Bring (experimental) Synapse support for delegated auth to an official release

[pmaier1]

• Admin API to provision users: Add a new admin API to create a new device for a user #15611
• Initial delegation support: Experimental MSC3861 support: delegate auth to an OIDC provider #15582
• Synapse reverse proxying to the auth server

We'll eventually want some follow-ups too:

• Appservice support
• Documentation
• Better configuration errors
• Handle TODO comments

[clokep]

@sandhose is taking care of this, but GitHub isn't letting me assign them.

[clokep]

This isn't actually done.