Way for API to announce any extensions they use in addition to Hydra Core

[tpluscode]

Introduce hydra vocabulary profiles or levels, i.e. level 0 and 1 where level 1 would be a default current hydra version and level 0 would be an alternative that does not imply a hydra:Resource is de-referencable (or event RDF based). This could also enable hydra for future profiles/levels using external schema vocabularies like SCHAL, OWL and non-rdf based ones.

Originally posted by @alien-mcl in #216 (comment)

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The third point however I find rather orthogonal. I'm not sure the "profiles" have anything to do with dereferencability of the API descriptions. More about the requirements from the client. I would see this as an open-ended set of "feature-packs" that a server may implement in addition to core. Here are some ideas:

1. Hydra Core

   • Only the base terms to put an API together

2. Standard Profile

   • hydra:Class

3. SHACL profile

   • Shapes for expects/returns
   • maybe also supportedClass

4. JSON Schema profile

   • like SHACL, uses a different vehicle to describe requests and responses

5. IANA Media types profile

   • Uses IANA-registered media types. for example the snippet below might denote an operation where PNG and JPG are allowed to be uploaded

   <> hydra:expects 
       <https://www.iana.org/assignments/media-types/image/png> ,
       <https://www.iana.org/assignments/media-types/image/jpeg> ,
   

6. Multi-part profile

   • Something we discussed in #199 where an operation is described as expediting a multipart/form-data body.
   • Individual parts could then be individually describe using the other profiles. For example to have one part image (IANA profile) and the other a plain RDF resource (Standard profile)

Each profile would be free to define its specific processing rules.

The required changes would be to:

1. Add an optional property on ApiDocumentation to assert which profiles a server uses
2. Relax some rdfs:range statements of extension points
3. Move the "Standard Profile" to a separate spec document. Would probably keep

So as effect a server might announce itself as

<> a hydra:ApiDocumentation ;
  // SHACL might potential be maintained by Hydra CG
  hydra:profile <http://www.w3.org/ns/hydra/profile#SHACL> ;
  // a vendor-specific profile which somehow bridges Open API
  hydra:profile <http://example.com/hydra-profile#OpenAPI>

Originally posted by @tpluscode in #216 (comment)

[alien-mcl]

I agree with the general idea - it was presented some time ago and it would solve a couple of the issues in this GH (i.e. #190 or #148).
The hydra:profile sounds promising - the only thing to decide on what kind of IRI should be here. Shall it be some custom URI pointing to extension or URI pointing just to vocabulary in use.

[alien-mcl]

Mayby a PREFER header could be also used to negotiate possible profiles and extensions where possible

[alien-mcl]

This should be resolved by PR #222 - closing