Canonical use of the @domain as a resource

[gsvarovsky]

The domain is a natural identity for a Subject with global properties, e.g. a security secret [2].

For some apps it may also be dereferenceable as a web resource [1].

At present in the Javascript engine the domain value is constrained to be a DNS domain name.

Suggested steps:

• For DNS-style domain names (i.e. those matching the DNS name regex), the canonical Subject name for the domain global Subject should be http://${domain}/ – i.e. the root resource with the HTTP protocol. Update: implemented in Whole domain authorisation m-ld-js#85
• In future, domain names should be derestricted to be any IRI. If a context @base is provided, the @domain is resolved against it. (Since a DNS domain name is not a valid IRI, point 1 continues to hold.)

[1] @gsvarovsky in #75 (comment)
[2] https://github.com/m-ld/m-ld-security-spec/blob/main/design/suac.md#ontology