[DAC] Make `raw_hash` encoding JSON only

The following discussion from !8496 (merged) should be addressed:

• @andrea.cerone started a discussion: (+3 comments)

  I'm afraid here you will need the old hash type, and still use the plugin module instance. This is because the binary encoding of a raw hash will add the 4 prefix bytes to the root hash, which would cause the external message deserialization to fail on the rollup kernel side.

  I wonder whether it's best to make the raw_hash_encoding json only to avoid these scenenarios at compile time. If you agree, better to do it in a follow-up