User API: support pulse gate calibrations

[SooluThomas]

Summary

This PR implements stage 2 after PR #4761

We introduce a new circuit method add_calibrations.

Details and comments

def add_calibration(self,
                     gate: Union[Gate, str],
                     qubits: Union[int, Tuple[int]],
                     params: List[Union[float, Parameter]],
                     schedule: Schedule) -> None:
        self._calibrations[gate_name][(tuple(qubits), tuple(gate_params))] = schedule

TODO

• Fix this: Currently, it works for custom gates but not on the standard gates because gate.name errors for standard gates.

[lcapelluto]

Given the comment below, that we have to optionally accept params (I forgot about the case where gate is a str), I'm not sure if it's better to do add_calibration(gate, qubits, schedule, params) or add_calibration(gate, schedule, qubits, params), or even schedule first...
@ajavadia what is the intuitive order, here?

[drewrisinger]

I really appreciated seeing the examples you used to have in the tests, would love to see an example in the release notes or documentation somewhere.

[drewrisinger]

Few notes on documentation clarity
EDIT: removed b/c already merged, wasn't paying attention.