Add MSG API hook to validate a message prior to returning buffer from SB

[jphickey]

Is your feature request related to a problem? Please describe.
If a user wants to add additional error checking within the header fields, such as a CRC, there is currently no easy way to validate the value as part of the receive buffer process.

Describe the solution you'd like
Add a hook to validate the message before returning from SB.

Additional context
This can be a no-op in the default build, as only the CMDs have a (weak) checksum, and that was never really enforced/checked in CFS anyway. So for backward compatibility the implementation should be permissive. But if the user has customized MSG with additional fields and logic, they could implement more validation using this hook.

Requester Info
Joseph Hickey, Vantage Systems, Inc.