[Design Doc] Batched Messages

[wilwade]

Blocked by #9, https://github.com/Liberty30/polkadot-experiments/issues/27

Notes:

• Batches messages might need to be handled differently than on chain messages in [Design Doc] On Chain Messages #9
• Or maybe not?
• The biggest difference is that these reference a batch file
• Do we need to have requirements that it be a parquet file?
• We should have requirements around where it can be stored (IPFS only for now? perhaps this is a different design doc?)
• Need to give the byte-length of the file for capacity transaction reasons as well as limiting super massive files or attacks