Support tokens without kid when using multiple oauth providers and/or resource servers (backport #11965) (backport #12171)

[mergify]

Proposed Changes

• Support configuration of signing_keys, default_key and algorithms on each oauth provider under oauth2.oauth_providers
• Do not delete statically configured signing keys when signing keys are loaded from a jwks endpoint. Instead, static signing keys are merged with those dynamically loaded, either via jwks endpoint or via rabbitmqctl. In case of a matching kid, the dynamic key overrides the static key
• Support tokens without kid when using multiple resource_server(s) and/or when using multiple oauth_provider(s) (This addresses a known issue)
• Add more test coverage

There is a doc PR accompanying this PR.

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This is an automatic backport of pull request #11965 done by [Mergify](https://mergify.com).

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This is an automatic backport of pull request #12171 done by [Mergify](https://mergify.com).