Make Oncall vendor agnostic, but more modular

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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 by reading Iris/src/vendors/__init__py and the Oncall/Iris vendor references where Slack is named. Trace how vendor modes are selected from configuration, then define completion as configurable Chat, SMS, Call, and E-mail vendors that can include Teams without hard-coded Slack references.

Written by the indexing model from the issue text.