FetchContent protobuf and use for FindProtobuf functions

Following
https://gitlab.kitware.com/cmake/cmake/issues/19875#note_705896 it is
possible to FetchContent protobuf. Is it feasible to use that protobuf
and it’s compiler for the protobuf_generate_cpp and
protobuf_generate_python functions of FindProtobuf?

Those functions expect a protobuf compiler for compilation, but the
compiler will only be available after the targets introduced by
FetchContent have finished compilation.

You can probably set the expected variables to some $<TARGET_FILE:> values. Those functions should be making custom commands rather than direct execution, so that should be fine.

Cc: @craig.scott