Define usage of HPKE and info structure to be used.

[GarethCOliver]

Resolve #692

This specifies how to user JOSE HPKE in OpenId4VP by defining a 'session_info' structure to be included in the info parameter.

It uses a byte structure, following the pattern JOSE HPKE uses for the recipient structure.

Open Questions:

• Should jwk hash be included (currently yes for parity with session transcript)
• Should the structure be hashed, as is done with session transcript (currently no).

[Sakurann]

WG discussion in #692 (comment) said no to jwkthumbprint and yes to hash. However, after a discussion in the WG, given the changes in the jose-hpke specification, we have a choice as a wg and there is a preference not to hash, but want to check first if widely used libraries restrict the length or not. we will come back to it next wg call.

WG discussion today: yes to use separator bytes.

[GarethCOliver]

Updated to remove thumbprint.

My review of documentation of existing HPKE implementations is that they currently don't enforce the recommended interop limit. Given the upcoming change to text to remove that implementation I think it's safe to leave as is (but will let @martijnharing chime in once he does his research).