Fix #2514, change CFE_MSG_Message from union to struct

[jphickey]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to GSFC-SoftwareRelease@mail.nasa.gov and copied cfs-program@lists.nasa.gov.

Describe the contribution
Having this abstract type as a "struct" makes it match the Command and Telemetry abstract types. Futhermore, it better conveys the intent that this is an abstract object and should not be directly used or accessed in other ways.

It may still be implemented as a union underneath (depending on how MSG module chooses to implement) but that is hidden from public API. In the case of the default MSG module implementation, there are just a handful of cases where it is accessed internally as bytes, and those are simple enough to do with a cast.

Fixes #2514

Testing performed
Build and run all tests

Expected behavior changes
API simplification

System(s) tested on
Debian

Additional context
There is a (very slight) possibility that some app code has used the union CFE_MSG_Message directly, and the change from union to struct will cause an error if any such code exists (but that could should be fixed anyway). None of the current CFS apps appear to do this, so its OK in that domain.

If any users have implemented a custom MSG module, this will require a matching/corresponding update in that implementation.

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