Fix #488, Pad msg headers to 64 bit

[skliper]

Describe the contribution
Fix #488 - pads headers to 64-bit so that CFE_SB_GetUserData will work for message structures with elements up to 64 bit

Testing performed
Added #903 (to catch errors from CFE_SB_GetUserData), built unit tests for both primary only and primary + extended headers, passed.

Expected behavior changes
For primary only config - telemetry header required to 64 bit boundary (affects all receivers)
For primary + extended config - command header required padding to 64 bit boundary (affects all senders)

System(s) tested on

• Hardware: cFS Dev Server
• OS: Ubuntu 18.04
• Versions: Bundle main + Update CFE_SB_GetUserData unit test to catch padding differences between header/payload #903 (only required for testing) + this commit

Additional context
Will require updates in cFS-GroundSystem, both cmdUtil, and cmd/tlm from the GUI

Third party code
None

Contributor Info - All information REQUIRED for consideration of pull request
Jacob Hageman - NASA/GSFC

[astrogeco]

CCB:2021-01-13 APPROVED

BREAKS everyone's command and telemetry.

Does not violate CCSDS since this goes in the secondary header.

Gets rid of implicit compiler padding.

See nasa/cFS-GroundSystem#148 and nasa/cFS-GroundSystem#145