DID: Decentralized identifiers (DIDs) (XLS-40): (#4636)

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.

src/ripple/protocol/impl/TER.cpp

@@ -119,6 +119,7 @@ transResults()
         MAKE_ERROR(tecXCHAIN_SELF_COMMIT,            "Account cannot commit funds to itself."),
         MAKE_ERROR(tecXCHAIN_BAD_PUBLIC_KEY_ACCOUNT_PAIR, "Bad public key account pair in an xchain transaction."),
         MAKE_ERROR(tecXCHAIN_CREATE_ACCOUNT_DISABLED, "This bridge does not support account creation."),
+        MAKE_ERROR(tecEMPTY_DID,                     "The DID object did not have a URI or DIDDocument field."),
 
         MAKE_ERROR(tefALREADY,                     "The exact transaction was already in this ledger."),
         MAKE_ERROR(tefBAD_ADD_AUTH,                "Not authorized to add account."),
@@ -191,6 +192,7 @@ transResults()
         MAKE_ERROR(temBAD_WEIGHT,                "Malformed: Weight must be a positive value."),
         MAKE_ERROR(temDST_IS_SRC,                "Destination may not be source."),
         MAKE_ERROR(temDST_NEEDED,                "Destination not specified."),
+        MAKE_ERROR(temEMPTY_DID,                 "Malformed: No DID data provided."),
         MAKE_ERROR(temINVALID,                   "The transaction is ill-formed."),
         MAKE_ERROR(temINVALID_FLAG,              "The transaction has an invalid flag."),
         MAKE_ERROR(temREDUNDANT,                 "The transaction is redundant."),