rabbitmq-auth-backend-oauth2: map additional_scopes_key to extra_scop…

[Hathoute]

Proposed Changes

Fix rabbitmq#12750 by introducing a synonyms map that will map additional_scopes_key to extra_scopes_source, instead of replacing extra_scopes_source in the schema which will be a breaking change.

There's probably a better way to handle this, as this is my first time writing erlang code, let me know what you think.

Types of Changes

• Bug fix (non-breaking change which fixes issue 'additional_scopes_key' ignored when configuring multiple oauth2 resource servers rabbitmq/rabbitmq-server#12750 )

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