Add identity to TerminatedFailureInfo

[stephanos]

What changed?

Add identity field to TerminatedFailureInfo.

Why?

To allow storing the identity of the caller that requested the termination; and allow clients to retrieve it.

Breaking changes

No

Server PR

NA - usage example:

&nexusoperationpb.OperationOutcome_Failed{
  Failure: &failurepb.Failure{
    Message: event.Reason,
	FailureInfo: &failurepb.Failure_TerminatedFailureInfo{
	  TerminatedFailureInfo: &failurepb.TerminatedFailureInfo{
        identity: event.Identity,
      },
    },
  },
}