Make Iris vendor agnostic, but more modular

Currently there are a handful of vendor modules included in the project and each one is referred to by name (See multiple instances of "Slack" throughout the Oncall / Iris tools). I've already began work on removing all instances of "Slack" from the tools, instead replacing the references with "Chat" (The Teams 'Slack Channel' is now 'Chat Override', this can be the teams slack channel, MS Teams Webhook, etc). That being said, I'm running into a bit of trouble with implementing an MS Teams Vendor (The Iris/src/vendors/__init__py doesn't like the mode name 'chat').

If, by default, Oncall and Iris were more modular (Aka, make a portion of the config set the preferred vendors for Chat, SMS, Call, E-mail, etc. and it would select the appropriate vendor script.)

`Vendors:`
` Call: twillio`
` SMS: twilio`
` E-mail: smtp`
` Chat: teams`

Start with Iris/src/vendors/__init__.py and the Oncall/Iris tools where Slack is referenced. Trace how vendor names and channel settings are selected, then determine how configuration should select vendors for Call, SMS, E-mail, and Chat, including an MS Teams vendor; the issue is complete when those channels are vendor-agnostic and the Teams configuration works.

Written by the indexing model from the issue text.