Allow specifying protoc location using an env var for rust codegen #22718
Description
What language does this apply to?
Rust
Describe the problem you are trying to solve.
Prost allows specifying the location of the protoc binary if it's not in the PATH: https://docs.rs/prost-build/latest/prost_build/index.html#sourcing-protoc
The rust message codegen doesn't support setting the location of the protoc binary using an env var or a config option. It would be good to have this ability.
Describe the solution you'd like
Support optionally specifying an env variable with the path to the protoc binary.
Describe alternatives you've considered
Additional context
In tonic, we would like to support souring protoc from an env var for the service codegen, however the message codegen also needs to support specifying the location of protoc for users to be able to generate service and message using a single build step.