feat: improved builder pattern for RequestUrl

[nanderstabel]

Description of change

#26 should be merged first.

Added RequestObject variant to RequestUrl enum. The three variants of the enum now reflect the 3 possible ways to transfer authorization request parameters with the Provider:

1. RequestUrl::Request: All request parameters are directly represented via an AuthorizationRequest
2. RequestUrl::RequestUri: The request parameters are encoded into a Request Object (JWT) and hosted at an endpoint. The only parameters that are initially transferred to the provider are client_id and request_uri, which represents the endpoint wher the provider can obtain the actual Request Object.
3. RequestUrl::RequestObject: In this case, the actual request parameters are also encoded into a Request Object (JWT), but now this JWT is directly transferred to the provider as a value for the request parameter alongside the client_id parameter.

More info: https://www.rfc-editor.org/rfc/rfc9101.html#section-5

Links to any relevant issues

#19

How the change has been tested

Adjusted some existing unit-tests that relied on RequestBuilder
Added test_valid_request_object unit test.

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