AT POP for Public Client based on broker

[rayluo]

This is the implementation of the internal work item, and this internal design doc.
Currently, this PR provides a low level API to obtain a Proof-of-Possesson (PoP) access token.

We may add more end-to-end test case in the future. And we may provide a high-level API.

The beta preview of this feature is available by pip install --ignore-installed git+https://github.com/AzureAD/microsoft-authentication-library-for-python.git@pop

[bgavrilMS]

@rayluo - when are you closing this?

[rayluo]

@rayluo - when are you closing this?

After we wrap up the July Focus tasks, we can probably squeeze this in before next big task.

Also, I heard that nowadays the MS Graph supports POP token. Is that true? If so, we might as well figure out a way to test it end-to-end before the shipment.

[bgavrilMS]

@rayluo - when are you closing this?

After we wrap up the July Focus tasks, we can probably squeeze this in before next big task.

Also, I heard that nowadays the MS Graph supports POP token. Is that true? If so, we might as well figure out a way to test it end-to-end before the shipment.

I am not sure about Graph, but SPO and EXO do.

[bgavrilMS]

Gentle reminder.

[xiangyan99]

Done with smoke tests and did not find regressions.