Wrap op (de)serialization methods in anonymous namespace

It's a known bug that older GCC is not happy with method specialization in
the enclosing (global) namespace:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56480

This CL wraps the generated specialization methods in the anonymous namespace
to make sure the specialization is in the same namespace as the class.

PiperOrigin-RevId: 258983181