Suport for 'request_uri' when response mode 'direct_post' is used

[hesusruiz]

Section 5.5 of openid-4-verifiable-presentations says:

5.5. Passing Authorization Request Across Devices
There are use-cases when the Authorization Request is being displayed on a device different from a device on which the requested Credential is stored. In those cases, an Authorization Request can be passed across devices by being rendered as a QR Code.

The usage of the Response Mode direct_post (see Section 6.2) in conjunction with request_uri is RECOMMENDED, since Authorization Request size might be large and might not fit in a QR code.

To enable future scalability of the complexity of Authorization Requests, VCVerifier must support 'request_uri'. To simplify implementation, it is better to always present Authorization Requests in QR codes using 'request_uri'.

This also reduces the size of the QR code with the current Authorization Request, and facilitates scanning by the Wallet and makes it more robust under different lightning conditions.