feat: experimental S3 cache backend

[lyoung-confluent]

Description

This PR updates the existing fanal S3 cache implementation added many years ago with various fixes/new tests and enables it's usage via the --cache-backend s3://... CLI flag. Specifically, I removed the .index objects which are not necessary because S3 is strongly consistent and took advantage of the HeadObject API for the MissingBlobs function. While this uses the AWS S3 SDK, it can be pointed at any S3 compatible API (ex: GCS, Cloudflare R2, etc) via the endpoint url configuration feature.

Related issues

• Close Support S3 as a cache backend #6731
• Close S3 fanal cache backend #3820

Related PRs

• feat: s3 cache backend #3819

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).