[Feature/Addition] Add methods for messages to Action Network connector

[mkrausse-ggtx]

There is a messages endpoint which allows the ability to access mass emails sent. Building a get_messages() and get_message() method would allow people to create analysis of emails outside of the Action Network platform. It can return body text, subject lines, etc. Allowing for complex analysis not possible inside the software itself.

Here is the API documentation: https://actionnetwork.org/docs/v2/messages

Detailed Description

Context

I created a similar process for EveryAction and use it in analysis of donations per email, etc.

Possible Implementation

Priority

Medium. I actually already built it and I am creating an issue to allow others to comment on it.

[shaunagm]

@NirTatcher is working on this (I can't assign them to the issue, for Github anti-abuse reasons I suppose)