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.

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• Bug fix (non-breaking change which fixes issue #NNNN)
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• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
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