Payment requirements in ACL docs

[michielbdejong]

Split out from #4.

I think we agree that we don't want a specific payment requirements syntax in WAC, but would instead use WAC+VC which is also useful for other use cases, particularly https://solid.github.io/authorization-panel/authorization-ucr/#group-membership-vc which is what we originally ran into with the Indico registration module, where listing all people who are allowed access to a given event would be prohibitive.

[michielbdejong]

Done this with acl:PayingAgent for now. Implemented it in NSS and wrote a test-suite test for it, which NSS now passes.

[michielbdejong]

See also the issue about this at the monetization-tests.
See also the issue about this at node-solid-server.