Change valid regions to enums

Problem to solve

At the moment we accept a string for a region (just like the SDK does), but it would be nicer to have enums of valid regions. Downside would be if the regions change we need to open up a merge request.

Proposal

Create enums for valid regions

More information

• https://www.twilio.com/docs/video/ip-address-whitelisting#signaling-communication