Allow stubbing join info to enable easy cross account joining from browser demo

[hensmi-amazon]

Issue #: None.

Description of changes: Often we (and possibly builders would find this useful) would like to join SDK meetings in different accounts using the browser demo that may not be using the serverless demo (or have a public endpoint). Previously we had the following workarounds:

• Deploy a serverless demo in every account and keep it updated. Use Meeting ID to join non-serverless created meetings.
• Hard code credentials into browser demo and recompile.

This change basically adds hidden (at least in additional options, where it is easily removed) logic to allow stubbing the join info. Also formatted demo.

Testing:

Joined a test meeting created in one account with a serverless demo in another account.

Can these tested using a demo application? Please provide reproducible step-by-step instructions. Not relevant, just demo changes.

Checklist:

1. Have you successfully run npm run build:release locally? y

2. Do you add, modify, or delete public API definitions? If yes, has that been reviewed and approved? n

3. Do you change the wire protocol, e.g. the request method? If yes, has that been reviewed and approved? n

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.