Use CFE_MSG_CMD_HDR_INIT macro in Message Action Table implementation

[skliper]

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.
Current raw buffer/command implementation in MAT table isn't portable across different endian systems and is somewhat challenging/messy to implement.

Describe the solution you'd like
Similar to nasa/SC#35, the message action table could use the CFE_MSG_CMD_HDR_INIT macro and real command types to simplify table implementation.

Suggestion:
Typedef a union that contains each of the message types in the table, then define the array w/ the MAT info and unioned element

typedef union {cmda, cmdb} cmdbuff;
typedef struct {mat_info, cmdbuff} mat_element;
then mat_element[X] = ... where cmdbuff.cmda can set the header w/ the macro and actual elements of command directly

Describe alternatives you've considered
None

Additional context
Similar possible approach with SCH

Requester Info
Jacob Hageman - NASA/GSFC