Draft: Create JsonString type

Due to an error when creating custom_json_operation, we should probably just check if it is possible to convert the string to json and not do the conversion.

When we check that this change is OK - we should probably change to this type in:

1. account_create_operation
2. account_update_operation
3. account_update2_operation
4. comment_operation
5. create_claimed_account_operation
6. escrow_transfer_operation

Please review: @mzebrak @mkudela