add additional sentence of the value of the signing of the request

[Sakurann]

imported from https://github.com/openid/oid4vc-haip-sd-jwt-vc/pull/122/files#r1859192744

discussed in the WG. seems to be support to allow both signed and unsigned request, to make this profile global.

For now, start with saying that wallets supporting HAIP MUST support both, and verifier has a choice and might have to support one of those if they need to based on the issuer policy.

Separate questions are:

• for EU, separate profiling will be required, if they want to make signing mandatory.
• Might want to add additional sentence of the value of the signing of the request