feat: Add SIOP Request features

[nanderstabel]

Description of change

This PR adds the following features:

• A new RequestUrl enum type with two variants: one for a request by value ( the SiopRequest), and another for a request by reference. This last variant is defined by the request_uri parameter in the request url.
• A RequestUrlBuilder which enables easy construction of request urls.
• A separate validate_request() method for Provider. This method extracts the SiopRequest from the RequestUrl and does some validation of the contents.
• Support for the registration parameter. This includes proper serialization and deserialization to and from literal formencoded request urls. The serde_urlencoded crate does not support serialization for non-primitive types (in this case, Registration is a custom struct with array fields).
• The decode method for the Validator trait which enables validating a JWT using a designated public key.
• The skeptic crate which enables testing of example code in markdown files.
• Improved the README example and the RelyingParty tests by utilizing a wiremock mock server.

Links to any relevant issues

fixes #12
fixes #13
fixes #14
fixes #16

How the change has been tested

• Tested successfully locally against the formed Sphereon Demo website.
• Existing unit tests are updating in order to parse raw request urls (including the siopv2://idtoken? prefix) to the new RequestUrl type. The division of the new Provider::validate_request() method is also reflected in the unit tests.
• Added multiple tests for valid and invalid RequestUrls of both variants.
• Added multiple tests for valid and invalid RequestUrlBuilders.
• Added a tests directory which includes a new skeptic.rs file for testing the rust example in the README file.
• Utilized the wiremock mock server for the README example and RelyingParty unit test.

Definition of Done checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[daniel-mader]

Awesome work! 🥳