'help' rpc commands autocomplete

Summary:
Adds autocompletion of the commands for when getting the help of a command by using help <command>

Backport of Core PR #9500
Completes T432

Co-authored-by: Jason B. Cox <contact@jasonbcox.com>

Test Plan: ninja check

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1932