Use a trait for remote ByteStore network implementation

[huonw]

This separates the store::remote::ByteStore coordination code from the gRPC REAPI implementation code, by:

• adding a store::remote::ByteStoreProvider trait
• moving the REAPI implementation into store::remote::reapi and implementing that trait

This is, in theory, just a lift-and-shift, with no functionality change, other than maybe changing some logging and when exactly a work-unit is created.

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

• generalising the action cache too
• implementing other byte store providers
• dynamically choosing the right provider for store::remote::ByteStore

The commits are individually reviewable:

1. preparatory clean-up
2. define the trait
3. move the REAPI code and implement the trait, close to naively as possible:
   • https://gist.github.com/huonw/475e4f0c281fe39e3d0c2e7e492cf823 has a white-space-ignoring diff between remote.rs after 1, and the reapi.rs file in this commit
   • there are some changes: making functions non-generic, and eliminating the work-units etc. (all the code that was left in remote.rs)
4. minor clean-up
5. more major clean-up: inline the method calls into the trait definition, so that they're not just fn method(&self, ...) { self.real_method(...) }:
   • as with 3, this is just a move, without changing the details of the code...
   • ...except for some minor changes like converting ? into .map(|e| e.to_string())? on the get_capabilities call in store_bytes