Update SAMPLE to better correlate with the naming conventions used
elsewhere and documented in the CFE naming conventions

- Put all global runtime data into a single "SAMPLE_AppData" object
- Name all global-scope identifiers consistently with a "SAMPLE_" prefix
- Do not use SAMPLE prefix for members already inside global
- Command handlers named to match the command they process
- Command handlers return an int32 status value
- Telemetry payload defined in a separate structure
- Member names in telemetry struct following convention in the
  CFE naming convention document.

Put the HkTlm buffer into a union to ensure it is aligned
appropriately for conversion to a CFE_SB_Msg_t type.