Get and use server-defined schemas via the /Schemas endpoint

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Feature request

Which Nextcloud Version are you currently using: master

Is your feature request related to a problem? Please describe.
Currently, the app has been tested to work with the SCIM Playground test server as well as with OpenProject. But since each service is allowed to customize/tailor the user and group attributes to their needs, the current client implementation is not portable/adaptable to other servers.

Describe the solution you'd like
Retrieving the expected schemas for each server via a GET /Schemas request would allow us to modify each API request so that they conform to the expected schema.

Describe alternatives you've considered
Preferably, if a server provided a schema with an ID of urn:ietf:params:scim:schemas:core:2.0:User, for example, then the user schema should be exactly the same as that defined by the SCIM core schema. In reality, though, this is not the case, and requesting all other external services to conform to a single schema is neither feasible nor scalable.

Start by locating the client code that builds SCIM user and group requests, then inspect how it could retrieve and represent the server-defined schemas from GET /Schemas. Compare the response with the existing fixed schema assumptions. Done means requests adapt to each server's returned schemas, including the core User schema case, with relevant tests covering the behavior.