p2p: pass strings to NetPermissions::TryParse functions by const ref

Summary:
these are "in" params that are not cheap to copy.

This is a backport of core#21996

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13335