Set User Agent Header for S3 Requests

[ajschmidt8]

Closes #2136.

This PR configures the S3 backend to use a custom HTTP client that has a user agent header set.

The user agent header enables sccache users to write AWS policies to accept or reject sccache requests to an S3 bucket based on their sccache version.

The user agent header value is like sccache/0.7.7.

[ajschmidt8]

hmm. i'll have to fix this import issue.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 30.49%. Comparing base (0cc0c62) to head (d9251b6).
Report is 16 commits behind head on main.

❗ Current head d9251b6 differs from pull request most recent head 96f7ed2. Consider uploading reports for the commit 96f7ed2 to get more accurate results

Files	Patch %	Lines
src/cache/s3.rs	33.33%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2137      +/-   ##
==========================================
- Coverage   30.91%   30.49%   -0.42%     
==========================================
  Files          53       53              
  Lines       20112    20486     +374     
  Branches     9755     9954     +199     
==========================================
+ Hits         6217     6248      +31     
- Misses       7922     8189     +267     
- Partials     5973     6049      +76     

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[Xuanwo]

This PR looks good to me now!

[Xuanwo]

cc @sylvestre, would you like to take another look?

[sylvestre]

thanks

[AJIOB]

@ajschmidt8 , just for note: do this really works in the prebuilt binary automatically (if you run the binary without cargo run)? Or we always need to define these Cargo variables externally?

[ajschmidt8]

@ajschmidt8 , just for note: do this really works in the prebuilt binary automatically (if you run the binary without cargo run)? Or we always need to define these Cargo variables externally?

rust's env! macro captures environment variables at compile time: https://doc.rust-lang.org/std/macro.env.html.

and Cargo sets these values at compile time.

so they won't need to be defined externally. it will work in the prebuilt binary automatically.

@AJIOB, just curious. do you intend to use this feature? if so, what's your use case?

[AJIOB]

No, I just confused about backward compatibility with existing usages of S3 backend.