Configure Oauth scope prefix

[MarcialRosales]

RabbitMQ OAuth2 plugin uses the resource_server_id :

• to validate audience (https://datatracker.ietf.org/doc/html/rfc7519#section-4.1.3)
• as resource parameter to authorize a user and get an access token (https://datatracker.ietf.org/doc/html/draft-ietf-oauth-resource-indicators-05#page-3)
• as the prefix for all RabbitMQ's scopes

Scopes do not necessarily must have a prefix. Prefixes are necessary, at least, in this situations:

• to avoid scope collision. For instance, in UAA, all scopes are defined in the same namespace so scopes must prefixed to avoid scope collision with other resources.
• to conform with Idp conventions. For instance, in Azure, all scopes must always carry a prefix which follows the pattern api://<app_id>..

Clearly, the scopes' prefix does not necessarily match either the audience claim or the resource parameter.

Proposed Changes

Therefore, there is a new setting called scope_prefix. It can be an empty string meaning that the scopes are the raw RabbitMQ scopes, e.g. configure:*/* or it can be any prefix. But it is very important to know that the final scope follows the format: <scope_prefix><rabbitmq_scope> . See that there are no separator character.

In order to keep existing configuration working without making further changes, RabbitMQ defaults scope_prefix to <resource_server_id>.. See that RabbitMQ maintains the dot character in the prefix.

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This PR is accompanied by a doc's PR