Add mechanism to discover an agent's preferred proxy

[csarven]

solid/vocab#26 outlines a use case where an agent wants to use a preferred or trusted proxy to read or update resources (without disclosing their application's true origin). Such feature also has the benefit to work around servers that can't be reached due to not participating in the CORS protocol.

There is implementation experience based on https://dokie.li/ where the application reads a WebID Profile's solid:preferredProxy (squatted ns) and routes fetch requests to target resources through its proxy. I currently use:

<https://csarven.ca/#i>
  solid:preferredProxy "https://localhost:8443/proxy?uri=" .

where I have a Solid server running on my local machine. I could use a remote server, i.e., the Solid server running on csarven.ca, but I didn't set ACLs on its proxy. This is orthogonal in any case.

So, irrespective to what the property is called or whether it uses a URI Template or some other way to convey the proxy URL and the part that an application needs to add in the URI of the target resource, the Solid WebID Profile specification should introduce this feature on some requirement level.

CORS is PITA (Pain in the Ass) and for Solid applications to take advantage of resources beyond those on a Solid server or a server that properly implements CORS, there needs to be use of a proxy. Otherwise, a whole category of use cases for Solid applications is a non-starter. Again, dokieli has plenty of experience on this - resources could be anywhere. And undoubtedly other applications have run into similar shortcomings that can be overcome by:

• Have Solid WebID Profile explain the discovery of agent's preferred proxy.
• Add property to Solid Terms.