Support self-hash value appearing in the subdomain

[vdods]

did:tdw supports DIDs whose "self certifying identifier" appears in the subdomain, e.g. did:tdw:{SCID}.example.com (see https://bcgov.github.io/trustdidweb/#method-specific-identifier).

did:webplus should be made to support this as well.

Comment: Because specification of subdomains involves modification of DNS records by the domain owner, it would be reasonable to consider a control over a subdomain-based did:webplus DID to imply control over the domain itself, and therefore could be considered to signify the identity of the VDR itself. This comment is not meant to necessarily establish that assumption in the did:webplus scheme.