[enhancement]: Implement OAuth Public profile and associated permissions

[filipnavara]

Which feature or improvement would you like to request?

I'd like to see this feature:

Commit 200d8d7 implemented the OAuth Dynamic Client Registration which is a necessary prerequisite to enable interoperability with 3rd party desktop email clients for OAuth authentication. A
draft profile is currently developed by IETF to specify additional requirements and tie the existing specifications together into an end-to-end solution. Stalwart currently doesn't implement the per protocol permissions AFAIK but it's pretty close to meeting all the other requirements.

Is your feature request related to a problem?

Trying to get an interoperable solution for OAuth with on-premise server deployments. (Working on the client side at @emclient)

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[zvasilev]

An very small but useful enhancement aligned to this.
Per both RFCs, login_hint is (optional). It's ok for email services.
For 3rd party email clients/services, this enhances initial auto-discovery and re-authentication.