[backport#12916] Introduce BigEndian wrapper and use it for netaddress ports

Summary:
This is another small improvement taken from #10785.

Instead of manually converting from/to BE format in the CService serializer, provide a generic way in serialize.h to serialize BE data (only 16 bits for now).

This is a backport of Core PR12916

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5799