dynamic metadata value

[mversic]

          I think the values in metadata should be reconsidered. I'd be in favor of loosely typing it with `StringWithJson` and defer the parsing of the structure to executor as we do for permission tokens payload. But this is something I think should be explored in a separate PR

Originally posted by @mversic in https://github.com/hyperledger/iroha/pull/4305#discussion_r1515996223