This change adds a protoc plugin which generates service code for pw_rpc
using nanopb as the protobuf implementation. Only the plugin code is
added here; it is not yet integrated into the build system.
Change-Id: Ia24bb5d130bf54c7afb41af2bca7f346e14bed9b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/12941
Commit-Queue: Alexei Frolov <frolv@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
