pw_rpc2: Move C++ code to cpp/

Move the C++ sources and their build targets into pw_rpc2/cpp/. The
top-level pw_rpc2 build files keep the docs and aggregate the cpp/ test
group, keeping the C++ implementation separate from the module docs and
any future implementations in other languages.

Change-Id: I45df39bc3760f9599b9b99fac4f9922c6729f027
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/484389