clarify the behavior when authorization_signed_response_alg is ommitted,

[Sakurann]

JARM spec says 'If unspecified, the default algorithm to use for signing authorization responses is RS256. The algorithm none is not allowed.' and I don't see that override in openid4vp (so if you don't specify it then it's RS256). but openid4vp spec makesauthorization_signed_response_alg optional because signing in JARM in openid4vp is optional. need to make clear that in openid4vp, ifauthorization_signed_response_alg` is ommitted, there is no default value, default is not signign

[bc-pi]

This needs to be fixed and I'll do a PR.

[bc-pi]

I did a PR! See #410