pw_protobuf_compiler: Allow protoc override

Normally pw_protobuf_compiler uses the protobuf compiler that comes as a
prebuilt from cipd. The Fuchsia build doesn't fetch pigweed's cipd
prebuilts but does actually build protoc itself.

This CL adds a gn two build args that allows a build to specify a target
for the protoc executable:
 pw_protobuf_compiler_PROTOC_TARGET
    The GN label to build the protoc binary.
 pw_protobuf_compiler_PROTOC_BINARY
    The path, relative to root_build_dir, of the built protoc binary.

Normal pw_protobuf_compiler usage is unaffected.

Change-Id: Iabf1058994cc501f995a8dbf536f2ed3222fc557
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/147310
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Ian McKellar <ianloic@google.com>
Pigweed-Auto-Submit: Ian McKellar <ianloic@google.com>