feat: add GCS support as file provider (#2013)

[Umang01-hash]

Pull Request Template

Description:

• Added GCS file provider in pkg/datasource/file/gcs .
• Implemented Create , Remove , ReadDir , Open ,Stat and MakeDir using cloud.google.com/go/storage .
• Simulates directories using object key prefixes .

Checklist:

• I have formatted my code using goimport and golangci-lint.
• All new code is covered by unit tests.
• This PR does not decrease the overall code coverage.
• I have reviewed the code comments and documentation for clarity.

Fixes #2013

NOTE: This PR is a continuation of PR #2117. Most of the work here belongs to Suryakantdsa. I only extended it to include common logger and metrics for File Systems.

[Umang01-hash]

@Suryakantdsa I have updated this PR with the Seek, ReadAt and WriteAt method implemented by you.

I think we have refactored almost all of the review comments given on PR #2013. And moreover GCS implementation is now using these common components. (logger, metrics).

@akshat-kumar-singhal Can you take a final review on this.

[Suryakantdsa]

Thank you so much @Umang01-hash for taking the time to review and implement the required GCS changes. Really appreciate your help!

[akshat-kumar-singhal]

We are in a good state structurally - some improvements have been shared. Summarising:

• Common capabilities should not be present in the file provider (gcs) package
• Establish consistency in the logging pattern in terms of log level and avoid redundancy

[akshat-kumar-singhal]

Most of the code here can be promoted to the file package as it's a common implementation, agnostic to the file provider

[Umang01-hash]

I have moved common utility functions (GetBucketName, GetObjectName to file/fs.go since they're provider-agnostic.

However, the actual directory operation implementations (Mkdir, ReadDir, Stat, etc.) remain in gcs/fs_dir.go because they contain GCS-specific logic (client calls, error handling, observability with GCS context).