Add -s option to enforce source bucket name

[h6ah4i]

Summary

This pull request introduces a new -s CLI option that allows specifying a bucket name directly instead of extracting it from the requested paths.

My use case

I'm hosting a private website with a private bucket protected by Identity-Aware Proxy (IAP).

flowchart LR
  http(http)---LB-with-IAP
  subgraph LB-with-IAP
    direction TB
    A[Identity-Aware Proxy].->B[Application Load Balancer]
  end
  LB-with-IAP---cloud-run[Cloud Run - gcsproxy]
  cloud-run---bucket[(Private Bucket)]  

The path rewriting feature provided by the load balancer can be used to pass the bucket name to gcsproxy.

However, this setup creates an issue when combined with IAP. When IAP is enabled, it processes requests after the path has been rewritten. As a result, the redirect URL after completing the authentication flow will include the rewritten path prefix, which is unintended. Using the new -s option instead of path rewriting resolves this issue.