Use a trait for remote action result caching

[huonw]

This separates the remote::remote_cache coordination code from the gRPC REAPI implementation by:

• adding a remote::remote_cache::ActionCacheProvider trait
• moving the REAPI implementation into remote::remote_cache::reapi and implementing that trait

This is, in theory, just a lift-and-shift, with no functionality change.

This is preparation work for supporting more remote stores like GHA cache or S3, for #11149, and is specifically broken out of #17840. It continues #19050. Additional work required to actually solve #11149:

• implementing other byte store and action cache providers
• dynamically choosing the right providers for store::remote::ByteStore and remote::remote_cache

The commits are individually reviewable:

1. preparatory breaking out of gRPC code
2. defining the trait
3. move the REAPI code and implement the trait, close to naively as possible:
   • https://gist.github.com/huonw/a60ad807b05ecea98387294c22de67cb has a white-space-ignoring diff between remote_cache.rs after 1, and the reapi.rs file in this commit (it's less useful than Use a trait for remote ByteStore network implementation #19050, since most of the code is deleted, but, buried in there are chunks of completely unchanged code)
4. minor clean-up

[huonw]

Friendly ping 😄