Resolves noir-lang/noir#4427

This PR just introduces the Call opcode to cleanly separate the breaking serialization change from its implementation. There will be followup work as specified under noir-lang/noir#4426 for implementing this opcode.

I ultimately settled on just having three fields for the opcode.

1. We just need an ID which acts as a function pointer that the execution environment will use for setting up the circuit that gets passed to the backend.
2. Inputs/Outputs just need to be witnesses as they should be treated the same way we treat inputs and outputs from any entry point function
3. I did not add a field for including metadata as I don't see the need. There is only two things a backend can really do with a function call. Either inline the function or fold the function. If a backend has the ability to fold is there a case where it would chose not to fold a function and would rather inline the function call? If we feel this is too restrictive I can add a metadata field. However, any metadata field should be an enum so any need to attach metadata to a function call would still require a breaking change.