remove Sealed trait from Instruction supertraits

[mversic]

To use client.submit() for CustomInstructionExpr, it is necessary for CustomInstructionExpr to implement Instruction and Sealed traits. This will require some changes to iroha_data_model (at least make Sealed trait public). I am not sure if it is worth it.

Currently I added pub fn into_instruction(self) -> InstructionBox method to CustomInstructionExpr, so it is possible to write:

let isi = CustomInstructionExpr::If(...);
client.submit_blocking(isi.into_instruction())?;

Originally posted by @dima74 in #4645 (comment)

[mversic]

also update the test from #4645 to reflect the new change