pw_protobuf_compiler: Handle deps on nanopb.proto

- Add a pw_proto_library for nanopb.proto to the CMake build.
- Skip recompiling nanopb.proto when running protoc for Nanopb.
- Add the pw_protobuf_compiler Nanopb test to CMake.

Change-Id: I3469f35524b18efd3d484b17609e9de0f160e73a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/46961
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>