Reduce profile list network traffic burden by making optional unless explicitly asked for

[rob-metalinkage]

The "cartesian product" form of profile listing can become quite onerous.

We should consider making it a "MAY" to return this for GET requests - and to ensure the mechanism is actually supported making it mandatory only if a specific named profile is asked for - so we'd need to mint that id.

(we could make it HEAD specific - but that violates HEAD=GET without body expectation..)

[larsgsvensson]

I don't think it's a good idea not to list the cartesian product. If we make it mandatory only when the client asks for a specific profile, then there is no way the client can figure out which profiles are available unless it asks for a dummy profile.

[rob-metalinkage]

Consider along with 300 response #5

Relax spec to say MAY provide complete list but MUST provide link to Altr profile.

This solve server side load to calculate all options.