Fix #81, use payload sub-structure

[jphickey]

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• [x] I reviewed the Contributing Guide.
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Describe the contribution Put all cmd/tlm message content in a sub-structure named "Payload"

Fixes #81

Testing performed Build and run CFE with LC, sanity check

Expected behavior changes No impact

System(s) tested on Debian

Contributor Info - All information REQUIRED for consideration of pull request Joseph Hickey, Vantage Systems, Inc.