[feature] S3 Backend / optimizations

[or-shachar]

Hi, excited to try this out! I already seen 90% reduction in my go CI times! ❤️ ❤️
I took the code and added support for S3 backend (PutObject/GetObject)...

Are you planning to maintain this repo? I can surely share my work and open a PR.

Few notes:

• I love the report of hit/miss/error. Reminds me of the good parts of Bazel.
• I think I'd prefer to have the concept of local cache and remote cache more structured. Local cache is the disk cache and it's the baseline. Remote cache can be either the http cache, S3 cache or something else. For dev machines in the final report - it'd be interesting to show how many hits were remote and how many were local.

[or-shachar]

I've advanced my fork since opening this PR... Feel free to check it out https://github.com/or-shachar/go-tool-cache/
I'll be more than happy to contribute it back.