Clarify intent_to_retain

[davidz25]

As discussed in WG10 meeting in Arizona, currently intent_to_retain is OPTIONAL but we don't say what value the wallet should use in this case. Suggest that we add "If not specified, the wallet shall use the value false".

[jogu]

Discussed on today's WG call: Consensus is a PR with a suggestion on the actual text would help people.

[jogu]

Discussed on today's EU call too: There is an additional option of explicitly stating that intent_to_retain being optional means that the verifier is not specifying if it will retain or not, which is arguably what the current spec says.

[jogu]

some discussion about this is recorded on openid/OpenID4VC-HAIP#233 too