Add FormatSubVersionUserAgent.

Summary:
Adds FormatSubVersionUserAgent, which takes the pre-formatted user agent.

Prerequesite for -useragent config option.

Test Plan: ./src/test_bitcoin -t util_tests

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9637