Bulk download and HTML requirements class

[heidivanparys]

If a Web API implements Requirements Class "HTML" from OGC API Features, then one or more enclosure links can have media type text/html.

See e.g. the implementation at https://demo.pygeoapi.io/cite/collections/?f=json (download is used instead of enclosure).

As a consequence, we should look at requirement /req/pre-defined/enclosure-type. This issue may also be related to #47 .