Proof of Possession (POP) token request

[rayluo]

• Adding an internal workitem link here as a placeholder. And another internal workitem for AT POP.
• The public specs on how to obtain a POP token, and another specs on why. And, although out of the scope of this feature requirement for this library, this is how a POP token will be used.
• An internal doc of AAD POP AT proposal.
• "For now, sending confirmation key req_cnf is supported on /token endpoint only. Only need to support asym keys, thought it is up to the client to format the key. The SDK should just pass it along and then use it to lookup cached tokens. Think if it as extra scope." -- Y. R.
• MSAL .Net's API

UPDATE on Nov 2022: On hold. New internal doc.

[andrewmwhite]

Is there any plan to implement this in the future?