RFD 211: Azure DevOps Joining (#54341)

* Outline RFD

* Update with examples

* Reorganize research & add more detail

* Add note on claims

* Add example script

* Start filling out overview

* Update rfd with spec

* Add missing comment

* Start to expand on security risk

* Add TODO for Join RPC

* Finish sentence regarding ID token validation

* Split alternatives and out of scope

* Add rough overview of validation steps

* Finish incomplete sentence

* Add some additional notes

* Addtional notes on issuer

* Clarify claims and issuer changed

* Update reviewers with gh handles

* Fill out alternatives and out of scope

* Appease cspell

Author: strideynet