Use a trait for remote ByteStore network implementation (#19050)

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`)
5. minor clean-up
6. 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`

src/rust/engine/fs/store/src/lib.rs

@@ -801,9 +801,7 @@ impl Store {
           ingested_digests.keys().cloned().collect()
         } else {
           remote
-            .list_missing_digests(
-              remote.find_missing_blobs_request(ingested_digests.keys().cloned()),
-            )
+            .list_missing_digests(ingested_digests.keys().cloned())
             .await?
         };
 
@@ -958,10 +956,7 @@ impl Store {
     } else {
       return Ok(false);
     };
-    let missing = remote
-      .store
-      .list_missing_digests(remote.store.find_missing_blobs_request(missing_locally))
-      .await?;
+    let missing = remote.store.list_missing_digests(missing_locally).await?;
 
     Ok(missing.is_empty())
   }