Bazel: remote/feature: support AWS S3 as a remote caching backend

[buchgr]

We have heard from many users who want to use AWS S3 as a remote caching backend for Bazel just as they currently do with Google Cloud Storage. This is useful when running inside Travis or CircleCI (https://github.com/notnoopci/bazel-remote-proxy) as these SaaS products already run inside AWS.

From skimming over the AWS S3 API documentation it seems that the APIs are mostly compatible, except for authentication. So implementing this feature will likely just mean to add AWS authentication to Bazel.

cc: @davidstanke @alexeagle

[GregBowyer]

4889 is a stab at this

[buchgr]

This is a duplicate of #4889

[keithkml]

Hi @buchgr, on this project, is a PR normally considered a duplicate of the feature request which requests it?