Add support for including comments in Zendesk source

[stevewb1993]

The default zendesk source does not (as far as I can tell) include comments which are added to tickets. It's quite easy for users of DLT to update the existing source code to include it if they read how to on the Zendesk documentation, but I think it would make it easier for DLT users if this was included as an option in the existing code.

Per my discussion on slack here, I'm raising an issue for this and will raise a PR shortly to implement this.

Are you a dlt user?
Yes, I'm already a dlt user.

Proposed solution
Per the zendesk documentation, use sideloading to include comments. This means adding "include": "comment_events" in the parameters to the /api/v2/incremental/ticket_events.json endpoint.
I'm open to suggestions on whether this should be included by default or not. My initial thought was to add a flag here for whether to include ticket comments, with a default value of False.

Apologies if I've raised the wrong type of issue for this. I wasn't sure if this counted as extending a source.

[stevewb1993]

draft pr for discussion here