[asmap] Make DecodeAsmap() a utility function

Summary:
DecopeAsmap is a pure utility function and doesn't have any
dependencies on addrman, so move it to util/asmap.

Reviewer hint: use:
git diff --color-moved=dimmed-zebra --color-moved-ws=ignore-all-space

This is a backport of core#22911 [3/4]
https://github.com/bitcoin/bitcoin/pull/22911/commits/9fd5618610e91e3949536c5122cf31eb58c9aa6b

Depends on D10780

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10781