rpc/net: decode the services flags in a new entry

Summary:
This adds human readable network services to the output of getpeerinfo and getnetworkinfo

Backport of Core PR16787

Test Plan:
Run src/bitcoin-cli getnetworkinfo and check the localservicesnames field.

More tests coming in PR16850

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7948