Proto,Tx_rollup: batch JSON encoding with hexadecimal

Context

No change in binary representation. This is needed because JSON strings can only contain unicode.

Otherwise the JSON encoded output will produce things like

{ "invalid_utf8_string" : [...] }

in place of the expected string.

Manually testing the MR

Run tezt test which checks that contents is a (valid JSON) string:

dune exec tezt/tests/main.exe -- tx_rollup batch encoding

Checklist

• [-] Document the interface of any function added or modified (see the coding guidelines)
• [-] Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• [-] For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR