Add AWS S3 storage backend

[dobesv]

Add support for cache storage to AWS S3.

[ecraig12345]

Thanks for this PR, but most of the current users of lage/backfill won't want a tree of AWS dependencies pulled in by default.

An alternative would be to specify the AWS deps as peerDependencies that are marked as optional in peerDependenciesMeta (and update the provider to throw if the dep isn't present). The peer deps would need to be specified in both the backfill-cache and backfill packages.

Also, due to the potentially-missing dependency and the way lage's types rollup works, it would be best to avoid directly references the types in backfill-config--so clientConfig?: object in S3CacheStorageOptions.

[dobesv]

• I moved the shared Azure/S3 classes to their own files so they can be shared between those two backends.
• I changed the types in config so they don't need to reference the aws-sdk package types