Function process_message_plaintext
pub unconstrained fn process_message_plaintext<ComputeNoteHashAndNullifierEnv, CustomMessageHandlerEnv>(
contract_address: AztecAddress,
compute_note_hash_and_nullifier: ComputeNoteHashAndNullifier<ComputeNoteHashAndNullifierEnv>,
process_custom_message: Option<CustomMessageHandler<CustomMessageHandlerEnv>>,
message_plaintext: BoundedVec<Field, 12>,
message_context: MessageContext,
)