compile well-known protobuf types in remote cache / execution code

[tdyas]

In the switch to Tonic, the "well-known" protobuf types (google.protobuf.*) are now provided by the prost-types crate. Binary fields in that generated code use Vec<u8> (which is the default in Prost). We should configure Prost to compile well-known protobuf types itself so that we can have binary fields use Bytes instead, which is more efficient.

Doing this is straight-forward but is blocked by a bug in Tonic documented in hyperium/tonic#521 and potentially resolved by hyperium/tonic#522.