[Feature] Add support for custom HTTP headers and URL path prefix

[cjoy]

What changes are proposed in this pull request?

Add two new configuration options to enable the SDK to work with HTTP proxies:

1. HTTPHeaders (map[string]string): Custom headers added to all API requests
2. HTTPPathPrefix (string): Path prefix prepended to all API URLs

These options can be configured via:

• Struct fields (programmatic)
• Environment variables: DATABRICKS_HTTP_HEADERS (format: key1=value1;key2=value2) and DATABRICKS_HTTP_PATH_PREFIX
• Config file attributes: http_headers and http_path_prefix

Files changed:

• config/config.go: Added new config fields with struct tags
• config/config_attribute.go: Added support for parsing map[string]string from environment variables
• config/api_client.go: Added request visitors for applying headers and path prefix

How is this tested?

Existing CI.

Also added unit tests in config/config_test.go:

• TestConfig_HTTPHeaders: Verifies headers are applied to requests
• TestConfig_HTTPPathPrefix: Verifies path prefix is prepended
• TestConfig_HTTPHeadersAndPathPrefix: Verifies both work together
• TestConfig_HTTPHeadersFromEnvVar: Verifies env var parsing for headers
• TestConfig_HTTPPathPrefixFromEnvVar: Verifies env var parsing for path prefix
• TestParseMapFromString: Verifies map parsing edge cases

[github-actions]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger:
go/deco-tests-run/sdk-go

Inputs:

• PR number: 1340
• Commit SHA: 86ef628bacdf4ed1d0ee5929ad511e6283b028aa

Checks will be approved automatically on success.