Add a "validated" entry in the full Ledger's Json.

Reviewers:
  @JoelKatz @donovanhide

This is in response to "blocker" RIPD-698.
Does this section:
https://github.com/rec/rippled/blob/ledger-validated/src/ripple/rpc/impl/LookupLedger.cpp#L170-L189
now belong here?