All CMD/TLM message should put content in a "Payload" sub-structure

[jphickey]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I reviewed the README file to see if the feature is in the major future work.
• I performed a cursory search to see if the feature request is relevant, not redundant, nor in conflict with other tickets.

Is your feature request related to a problem? Please describe.
To match the patterns used in CFE and other modules, all CMD/TLM message definitions should put the content (non-header) parts into a separate struct called "Payload".

Describe the solution you'd like
Separate message content into a sub structure called "Payload".

Additional context
This is benefit to tooling that can use the presence of this field to identify where the actual content starts (e.g. something like offsetof(MsgType, Payload) would work and be correct, as opposed to checking sizeof(CFE_MSG_CommandHeader_t) which may not actually reflect where the content starts due to possible compiler-added padding between them).

Requester Info
Joseph Hickey, Vantage Systems, Inc.