Keep IRIs opaque to the client with iriPrefix

I was thinking about what we are missing to support use of IRIs opaque to the client. Main concern seems to be related to creating new resources. Looking at:

https://solid.github.io/data-interoperability-panel/specification/#datareg-create-instance

We could add two rather small changes to it:

1. Data Registration must provide IRI template or just base for creating new resource IRI, even something simple like void:uriSpece

2. PUT request creating new resource must have additional link in a header designating target Data Registration rel=http://www.w3.org/ns/solid/interop#targetDataRegistration

GIven above client and serve should still be able to create new resources in specific data registration. While it still allows server to use specific IRI hierarchy it doesn't require it.

Start with the Data Registration create-instance section in the linked Data Interoperability specification. Determine how a registration can provide an IRI template or base and how a creating PUT can identify its target registration with the proposed link relation. Done means the specification defines both changes consistently and explains how clients and servers use them.