Format-specific Credential Issuer Metadata: `value_type`: image is a string

[danielfett]

For W3C credentials, the field value_type is defined as

String value determining the type of value of the claim. A non-exhaustive list of valid values defined by this specification are string, number, and image media types such as image/jpeg as defined in IANA media type registry for images (https://www.iana.org/assignments/media-types/media-types.xhtml#image).

I suppose that image/jpeg is a subtype of string and there is some encoding applied. If that's correct, both facts should be made explicit in the spec.

[Sakurann]

i think this is related to #90 (cc @jogu :D)

[jogu]

The same text also appears in the ISO mDL section.

I think the 'A non-exhaustive list of valid values defined by this specification' part of the sentence is very weird too, as I can't find anywhere else where this spec defines values.

We also don't use value_type in any of the examples. We probably should...