rpc: add network field to rpc getnodeaddresses

Summary:
Add network field to rpc getnodeaddresses.

Simplify/constify code.

Improve help.

This is a backport of core#21594
Depends on D11125

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11126