[sso/oidc] Proto definitions for OIDC ProviderConfig in public-api

[AlexTugarev]

part of Epic: Single sign-on (SSO) #7761

The Provider Config entity is to describe an instance which connects to an IdP's client counterpart.

The CRUD service should allow us to manage those entries.

Out of scope:

• further automations, e.g. update to clients, invalidation, etc., to be added later on.
• let's make no strong assumptions about ownerships of those provider configs. different types of owners should be possible.

Notes:

• ProviderConfig entity can be a superset of oidc.ProviderConfig or oidc.RelyingParty
• Derive useful additional attributes from existing AuthProviders, e.g. descriptions, icons, scopes, hints
• Add CRUD service