Refactor environment variable implementation

[adreed-msft]

Description

In the course of work on the cap-mbps work item, I wanted to deprecate the old AZCOPY_CONCURRENCY_VALUE environment variable in favor of a more descriptive AZCOPY_CONCURRENT_ROUTINES variable. However, we don't have any real support for doing so. The intent of this PR is a few things.

1. Refactor the enumerator portion of environment variables to not be directly attached to an individual EnvironmentVariable.
2. Refactor the environment variables' interface to be called from the EnvironmentVariable directly, as opposed to calling a function with the environment variable as a parameter. This makes it less frustrating to work with an environment variable.
3. Support deprecating an environment variable, falling back to it's deprecated alternatives, warning about deprecated variables being set or read (in the latter case, panicking to alert a developer and enforce code quality).
4. Introduce ways to distinguish "" vs unset
5. Introduce ways to check the name of which deprecated environment variable had been used.

Type of Change

• Bug fix
• New feature
• Documentation update required
• Code quality improvement
• Other (describe):

How Has This Been Tested?

Refactor that shouldn't add any functionality-- existing testing should catch issues.

[Copilot]

Pull request overview

This pull request refactors the environment variable implementation to support deprecation, better distinguish between unset and empty values, and provide a more ergonomic API. The changes move from a global function-based approach (GetEnvironmentVariable(env)) to method-based access (env.Value()). The new EnvironmentVariable type includes support for deprecated variable names via Replaces and ReplacedBy fields, with automatic fallback and warning mechanisms.

Changes:

• Introduces new methods on EnvironmentVariable: Lookup(), IsSet(), Value(), Clear(), and LookupName() for accessing and managing environment variables
• Adds deprecation infrastructure with automatic warning generation when deprecated variables are used
• Updates all environment variable access throughout the codebase to use the new API pattern

Reviewed changes

Copilot reviewed 35 out of 35 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
common/environment.go	Core refactoring: adds new methods, deprecation support, and changes from function-based to method-based API
common/oauthTokenManager.go	Updates OAuth token manager to use new Lookup() pattern for checking if environment variables are set
common/version.go	Converts UserAgentPrefix access to use new Value() method
common/logger.go	Updates DisableSyslog check to use Lookup() with proper boolean handling
common/init.go	Refactors folder initialization to use Lookup() for distinguishing unset vs empty paths
common/init_windows.go	Updates UserDir access to use Value() method
common/init_unix.go	Updates UserDir access to use Value() method
common/credentialFactory.go	Updates credential creation to use Lookup() for checking variable presence
common/credCache_windows.go	Updates token cache file path logic to use Lookup()
common/clientFactory.go	Updates shared key credential creation to use Lookup()
common/ProxyLookupCache.go	Updates proxy cache lookup to use Value() method
ste/xferVersionPolicy.go	Updates default service API version initialization to use Value()
ste/xfer.go	Updates request timeout initialization to use Lookup()
ste/xfer-remoteToLocal-file.go	Updates download temp path check to use Value()
ste/sourceInfoProvider-GCP.go	Updates GCP credentials path access to use Lookup() with proper error messages
ste/sender-blockBlob.go	Updates blob transfer resume check to use Value() with strings.EqualFold
ste/pageRangeOptimizer.go	Updates sparse page blob optimization check to use Value()
ste/pacer-autoPacer.go	Updates pace page blobs check to use Value()
ste/concurrency.go	Updates concurrency and bool configuration helpers to use Lookup()
jobsAdmin/init.go	Updates MIME mapping path check to use Lookup()
jobsAdmin/JobsAdmin.go	Updates buffer GB override check to use Lookup()
traverser/zc_traverser_gcp_service.go	Changes projectID initialization to package-level with Value()
traverser/zc_traverser_blob.go	Updates hierarchical scanning check to use Value()
testSuite/cmd/platdiff_windows.go	Updates UserDir access to use Value()
testSuite/cmd/common.go	Updates AWS credentials access to use Lookup() with proper error messages
cmd/uihooks.go	Updates CPU and memory profiling path checks to use Lookup()
cmd/root.go	Updates concurrency value check to use Lookup() for auto-tuning logic
cmd/login.go	Updates certificate password and client secret checks to use Lookup()
cmd/env.go	Adds deprecation warning display when listing environment variables
cmd/copy.go	Updates concurrency check to use IsSet()
azcopy/output.go	Updates performance state display check to use IsSet()
azcopy/credentialUtil.go	Updates credential type and cloud provider credential checks to use IsSet() and Lookup()
azcopy/copyRedirection.go	Updates concurrency environment variable access with Lookup() for better error messages
azcopy/copy.go	Updates concurrency check to use IsSet()
azcopy/client.go	Updates login cache name check to use Lookup()

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[wonwuakpa-msft]

It should be good after addressing the copilot comments