Support vocabularies which use a method other than content negotiation to direct to their JSON-LD context

[elichad]

Some vocabularies provide the JSON-LD using a link header with rel=alternate; type="application/ld+json":

https://www.w3.org/TR/json-ld11/#alternate-document-location

Schema.org is one of these (see schemaorg/schemaorg#3500). For example, if you had the crate with context (ignoring the redundancy of including schema.org here):

"@context": ["https://w3id.org/ro/crate/1.1/context","http://schema.org"],
...

this error will appear

[ ro-crate-1.1_2.2 ]  File Descriptor @context property validation:                                                                              
                             Check if the file descriptor contains the @context property and it is a valid JSON-LD context                              
         Detected issues                                                                                                                                
         - [Violation]: Unable to retrieve the JSON-LD context "http://schema.org"     

This is because content negotiation for JSON-LD is not supported - e.g. the following command won't return JSON-LD as you might expect (and I think the validator currently only does something equivalent to this):

curl -H "Accept:application/ld+json" -L http://schema.org/

Supporting the alternate document location allows the context to be retrieved.

Schema.org is the example here, but this could be relevant in other cases too.