Oauth2 add support for UMA 2.0 flow

We see a common request for better security and privilege escalation in applications. Oauth2 providers have developed a few extra flows to address this kind of security requirements. One example is UMA 2.0:

https://justinsecurity.medium.com/uma-2-0-437c293c3283

We should add support for this flow to the list of existing ones.

@lusoalex I believe you already did some work related to this flow. Is there anything public you can share?