Add real-time Var support for ForLoopOp range

[arthurostrauss]

What should we add?

Currently,

the ForLoopOp requires passing an explicit list of integer (or a range) that specifies at compile time what will be the set of iterations. It would be nice to have the ability to create a custom range that could be specified at runtime to avoid compilation overhead when switching the number of repetitions of a same circuit.

This would involve creating an extra real-time range object (perhaps the opportunity to add arrays of real-time variables) that could serve as input for the operation.

One example of workflow would be:

`num_qubits = 3
q = QuantumRegister(num_qubits, "q")
qc = QuantumCircuit(q)
n_reps = qc.add_input("n_reps", Uint(8))

with qc.for_loop(range(n_reps)):
qc.x(0)
`
where here it is clear that the code will yield an error because range() cannot be applied over n_reps, hence motivating the introduction of a custom range object (similar to OpenQASM3) that could support such input.