Use protozero in .gen.cc proto->C++ classes

This CL is the major step towards geting rid of the
libprotobuf runtime dep (see go/perfetto-libprotobuf).
It switches the .gen.cc files to uses protozero for the
proto<>c++ conversions.
As-is it still support the legacy conversion from
libprotobuf-lite classes, but that is done going via
raw binary. So FromProto() now goes
pblite -> raw bytes -> pbzero decoder and ToProto() goes
pbzero -> raw_bytes -> pblite.
Effectively this doubles the round-trip for these two methods.
The next CLs will get rid of the FromProto/ToProto methods
and will switch the IPC layer to just use the .gen.cc classes
instead of the pblite ones.

Bug: 132880619
Change-Id: I721876c95dfa987b9aaa5a1009eeb64baa7e37dc