Fix resolving type metadata from URL and limit to HTTPS URLs

[awoie]

Current text:

In an SD-JWT VC, a URI in the vct claim can be used to express a type. If the type is a URL, metadata can be retrieved from the URL https:///.well-known/vct/, i.e., by inserting /.well-known/vct after the authority part of the URL.

Since well-known only applies to HTTPS URLs, we should limit the section to HTTPS URLs although other URLs would be still possible, e.g., ipfs://....

Additionally, also the <type> in https://<authority>/.well-known/vct/<type> is probably not correct since the type refers to the vct value itself which would not make sense to the full vct value here which would also include the scheme for instance.

I suggest we update the title to "From an HTTPS URL in the vct claim" and update the language to something like this:

If the vct value is an HTTPS URL, Type Metadata can be retrieved from the URL constructed by inserting "/.well-known/vct" after the authority part of the URL in the vct value. For example, a vct value of "https://example.com/foo/bar/mytype" would result in "https://example.com/.well-known/vct/foo/bar/mytype".