Add lightweight integrity check when constructing a DP container

[bocchino]

Do this after merging #901. That change moves the data hash generation out of the code that sets up the container. For more robustness, we can do the following:

1. Add a new constant CONTAINER_SYNC_WORD to config/DpCfg.fpp.
2. Add this as a framework constant in the FPP spec and implementation.
3. Add this as a required dictionary constant in the dictionary spec and FPP implementation.
4. Update the DP code generation to write this value into the data hash slot when constructing a DP container.