Allow Specifying Alternative RPC URL

[nlordell]

When a deployment PR is created, we use the first URL in the chainlist entry. It is not uncommon that this URL is down, but a working alternative URL exists in the chainlist entry.

I propose we add an optional:

Chain RPC:

text to the issue template that can be filled one of the other chainlist URLs should be used. The script MUST check that the manually provided RPC URL is part of the chainlist entry (and does not accept arbitrary URLs).