util: Add Join helper to join a list of strings

Summary:

• util: Add Join helper to join a list of strings
• rpc: Use Join helper in rpc/util

This is a backport of Core PR16670

Depends on D6376

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6377