C2MS — Add Command Echo message

Rich Monteleone of RTLogic suggested adding a command echo message. Here is an email from him about it. I (Jay Bugenhagen) also have available Rich's sample schema and some other info re: the potential new message.
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Per our discussions recently I’ve created a draft for a Command Echo GMSEC Message Schema (GMSEC.MSG.CMD.ECHO) for consideration by NASA/EGS.

Notes/Rationale:
Since Command Echo is executed out-of-band with Command Request/Response (GMSEC.REQ.CMD) I’ve elected to construct the schema for publish/subscribe messaging.
I based the schema’s off of the 2016.00 spec delivered with the GMSEC 4.3 API
I also attached the GMSEC.REQ.CMD schema, I’ve added an optional field that we found we needed for commanding (see CMD-ENCODING).

Discussion topics: We need to determine whether or not we should add the comparison of metadata associated with a commands to command echo. Right now we are not comparing command metadata.
Example metadata (SPACECRAFT-EXECUTION-TIME, LATEST-UPLINK-TIME, EARLIEST-UPLINK-TIME, RELEASE-TIME)

There is a need to be able to echo commands from front-end equipment. The addition of this message allows for that capability.

Draft originally from Rich Monteleone (RTLogic). Eric Ogren supported final development of new message.