feat: implement user schema management (#7416)

This PR adds the functionality to manage user schemas through the new user schema service.
It includes the possibility to create a basic JSON schema and also provides a way on defining permissions (read, write) for owner and self context with an annotation.

Further annotations for OIDC claims and SAML attribute mappings will follow.

A guide on how to create a schema and assign permissions has been started. It will be extended though out the process of implementing the schema and users based on those.

Note:
This feature is in an early stage and therefore not enabled by default. To test it out, please enable the UserSchema feature flag on your instance / system though the feature service.

go.sum

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