QSA Profile Conneg realisation must support QSA discovery

[nicholascar]

If the QSA realisation of the profile conneg Abstract Model (https://w3c.github.io/dxwg/conneg-by-ap/#abstractmodel) doesn't mandate the use of particular QSA keys (such as _profile & _mediatype) then it needs to provide a way for users to discover what QSA keys are used.

[akuckartz]

I am not a huge fan of QSA, but I suggest that this discovery should also work for mandated keys.

[nicholascar]

@akuckartz a standardised function would return all the keys that an implementation uses for profiles, Media Types, etc. and this would be required to be implemented for all QSA implementations, even ones that use the "standard pattern" keys of _profile & _mediatype. I already have many API instances that use _view & _format.

This function would really be the starting point function that any implementation accessor could use to work out how the implementation is configured.

In HTTP land, there is no need for this function as the Accept-Profile request and the Content-Profile header keys will have been registered, just as Accept and Content-Type already are.

[nicholascar]

Are perhaps Well-Known Uniform Resource Identifiers (URIs) something worth considering?

[nicholascar]

This is being addressed in PR #1026