This amendment fixes an edge case where an empty DID object can be
created. It adds an additional check to ensure that DIDs are
non-empty when created, and returns a `tecEMPTY_DID` error if the DID
would be empty.
Implement native support for W3C DIDs.
Add a new ledger object: `DID`.
Add two new transactions:
1. `DIDSet`: create or update the `DID` object.
2. `DIDDelete`: delete the `DID` object.
This meets the requirements specified in the DID v1.0 specification
currently recommended by the W3C Credentials Community Group.
The DID format for the XRP Ledger conforms to W3C DID standards.
The objects can be created and owned by any XRPL account holder.
The transactions can be integrated by any service, wallet, or application.
