Return mapped AS in RPC call getpeerinfo

Summary:

If ASN bucketing is used, return a corresponding AS
used in bucketing for a given peer.

Partial backport 3/4 of core PR16702:
https://github.com/bitcoin/bitcoin/pull/16702/commits/e4658aa8eaf1629dd5af8cf7b9717a8e72028251

Depends on D8195.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8196