Add unanchored DID string ABNF

[troyronda]

When a DID is initially created, the Orb server doesn't yet have the CID related to the anchor graph.

This DID is considered unanchored and should have a DID prefix that distinguishes it from the anchored version. Once the DID becomes anchored, the Orb server will return the canonical DID that includes the CID. The client should try resolution until they receive the canonical version.

Note: Without the "long form" information, this DID can only be resolved locally against the Orb server that created it.

Possible prefixes:

• did:orb:local:
• did:orb:unanchored:

[troyronda]

This was initially added as did:orb:interim.