Support request_uri and validation

[nanderstabel]

Description

Fork the Sphereon demo website and alter some variables so that it suits our needs (alter the default DID method to did:key, add test private keys, etc) and so that it can be added as an integration test.

The demo website follows the following steps:

• A redirect_uri is presented in the form of a QR Code (by the relying party).
• The Provider needs to follow this uri in order to receive the JWT encoded SIOP Response.
• The Provider needs to decode and validate this JWT.
• The Provider generates a SIOP Response which includes a (JWT encoded) id_token and sends it to the Relying Party's redirect_uri

In order for this integration to work some Request parameters need to be added as well, such as the request_uri and the registration parameter.

Motivation

In order to test the validity of the library, we need to test the interoperability with other SIOP implementations.

Resources

Depends on #2
Sphereon demo website

To-do List

• Implement the request_uri parameter #12
• Implement JWT decoding and validation for the Provider struct #13
• Support the registration parameter #14