Extend purpose field for DCQL

[cre8]

This is not something for 1.0, but maybe 1.1 to extend DQCL usage

In the context of OID4VP, the purpose is a string which is fine for multi language support (using the information from the request to insert the correct translation on demand).

In our case we want to include the DCQL inside JWS, where it is static. It would be great to give multiple strings for the purpose to prevent the wallet to translate it by itself (and in the worst case in this happens in the wrong way). One suggestion to extend it in the future would be:

[
    {
        "locale": "en-US",
        "name": "Required for checking the minimum age",
    },
    {
        "locale": "de-DE",
        "name": "Benötigt für die Überprüfung des Mindestalters"
    }
]

• locale: REQUIRED: String value that identifies the language of this object
  represented as a language tag taken from values defined in BCP47 RFC5646.
  Multiple display objects MAY be included for separate languages. There MUST be
  only one object for each language identifier.
• name: REQUIRED: String value that defines the purpose.

To not break existing usage, I suggest the type of purpose would be string and this type.

[c2bo]

Related issue: #289