[Interoperability][JAR] Authorization Request Object requirements for interoperability

[Zicchio]

The interoperability requirements for the Authorization Request (which, in the case of the cross device flow, is the content of the QR code) has some differences with the current implementation. If we want to comply with potential, then we should find a way to satisfy those requirements.

Currently, the differences in the request object compared to the current implementation are:

• the request object jwt should include an aud claim with value equal to "https://self-issued.me/v2"
• client_id_scheme is omitted as no trust framework is used in potential
• presentation_definition is inserted directly in the request object as no trust framework is used in potential
• response_mode should be direct_post (this is related to [Interoperability] Support for unencrypted direct_post authz response mode  #310) as no trust framework is used in protential

I leave myself the option to edit the message is order to include further requirements relating the request object if or when they are observed.

[Zicchio]

Part of this issue includes #290