Fix #15, Use CFE_MSG_PTR instead of &(x).Msg

[thnkslprpt]

Checklist

• 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

• Fixes Apps should use CFE_MSG_PTR macro instead of cast or local unwrapping #15
  • Local unwrappings replaced with the abstracted CFE_MSG_PTR macro
  • Minor related naming convention updates:
    • CmdHeader renamed to CommandHeader
    • TlmHeader renamed to TelemetryHeader

Testing performed
GitHub CI actions all passing successfully (incl. Build + Run, Unit/Functional Tests etc.).

Expected behavior changes
No change.

System(s) tested on
Debian GNU/Linux 11 (bullseye)
Current main branch of cFS bundle.

Contributor Info
Avi Weiss   @thnkslprpt

[jphickey]

Thanks - we should merge this one