Create CFE_ES_HousekeepingCmd_t typedef to maintain app-specific command definitions

[astrogeco]

Is your feature request related to a problem? Please describe.
As part of the message processing pattern, applications currently define a header file (typically ending in _msg.h) that define the structures which serve as I/O message.

Notable exception to this pattern is in the "send housekeeping" commands - this has no app-defined type, it directly uses the type provided by the MSG module e.g. CFE_MSG_CommandHeader_t.

Describe the solution you'd like
For consistency in operation we should really make an equivalent app defined message type (e.g. CFE_ES_HousekeepingCmd_t) for this - it can be a simple typedef to CFE_MSG_CommandHeader_t. Having a type for this would maintain the pattern of having a dedicated typedef for each command definition that is locally defined/controlled by the app itself.

Additional context
App can still use or depend on types provided by other modules - this is OK - the main thing is that it shouldn't be assumed to do so. The SEND_HK style commands are unique in that they assume use of a bare CFE_MSG_CommandHeader_t for this message. This is fine but the app should say this somewhere, it shouldn't be assumed.

Having an app fully define its own I/O interface (with no assumptions) is also important if/when transitioning to a data dictionary/EDS type message system.

Requester Info
Joseph Hickey, Vantage Systems, Inc.

Originally posted by @jphickey in #1015 (comment)

[thnkslprpt]

These already exist now (XX_SendHkCmd_t) - I think this issue can be closed.