Add BackendV2 mocked backends

[mtreinish]

What is the expected enhancement?

The mock backends in qiskit/test/mock/backends are built using snapshots of IBM Quantum's devices. Right now they're built using BackendV1 (and BaseBackend for the legacy backend classes). To start fully testing the BackendV2 interface we should add variants of these fake backend classes using BackendV2. We shouldn't replace the BackendV1 objects since it's useful to exercise both versions of the interface while both are supported.

[HuangJunye]

I am interested in working on this issue. @mtreinish Can you elaborate what's needed to be done here?

[mtreinish]

It's basically to add a new base class similar to the current 3 built with backendV1 (https://github.com/Qiskit/qiskit-terra/blob/main/qiskit/test/mock/fake_backend.py#L47 https://github.com/Qiskit/qiskit-terra/blob/main/qiskit/test/mock/fake_qasm_backend.py#L29 and https://github.com/Qiskit/qiskit-terra/blob/main/qiskit/test/mock/fake_pulse_backend.py#L23) that is built using the backendv2 interface. This will basically involve passing the json payloads we store and creating a Target and BackendV2 object from it.

There is documentation on this here: https://qiskit.org/documentation/apidoc/providers.html#writing-a-new-provider and an example of hand constructed ones (as opposed to those built from parsing json) here: https://github.com/Qiskit/qiskit-terra/blob/main/qiskit/test/mock/fake_backend_v2.py and https://github.com/Qiskit/qiskit-terra/blob/main/qiskit/test/mock/fake_mumbai_v2.py

[HuangJunye]

Thanks for the explanations. This sounds easy enough for me to tackle. If this is not too urgent, I can try get it done by the end of Jan. Please assign me to the issue.