OAuth2: support opaque tokens

[MarcialRosales]

Proposed Changes

Implements #8662

RabbitMQ will never cache the resolved JWT access token. it will only be cached for the duration of the session/connection. Once RabbitMQ resolves the JWT access token (i..e exchanged the opaque one for a JWT one), it will work as usual, i.e. it will use the expiry date in the resolved JWT access token.

Tasks:

• Add introspection endpoint settings to schema
• Support opaque tokens via messaging protocols like amqp
• Add Selenium messaging test to assert opaque tokens are supported over amqp
• Support opaque tokens in management ui. It requires introspecting the token before passing it to the oauth backend. Otherwise, every single authorization request requires a token introspection
• Support opaque tokens when using idp-initiated login. Here opaque tokens arrive on the Authorization header and they need to be introspected too.
• Add Selenium tests for management ui with opaque tokens for sp-initiated login
• Add Selenium tests for management ui with opaque tokens for idp-initiated login
• Update documentation

Types of Changes

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• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI