For cross-compilation, we may not want to re-use the same protoc which would be used during a CMake build (these would differ in hostPlatform).
Instead, allow for invocations of `protobuf_generate` pass their preferred programs, similar to how you can do this for grpc plugins.
Optional proposal:
Use `find_program`, however, this is a pretty big behavior change, this PR allows you to "opt-out" of the legacy behavior, but users shouldn't see a difference unless they opt-in to passing their own protoc-exe command/path
Closes#17888
* doc: Add document that describes protobuf_generate
How to use it.
How it works. (basic and with gRPC)
Reference documentation.
* doc: Address several typos and wording issues in cmake_protobuf_generate.md
Jonathan Ringer
Dennis