Return only default backends by the Qiskit provider

[ArthurKamalov]

The qiskit's 'provider.backends' method returns only default backends.

Things to consider:

• In some cases, due to other filtering conditions, there may be resulting lists without any default backend.
  Like in this test -

  azure-quantum-python/azure-quantum/tests/unit/test_qiskit.py

  Line 573 in 2fd08fe

  backend = provider.get_backend("ionq.simulator", input_data_format="qir.v1")

  
  In such case we just do not filter for default backends and return previous version of the list.

• There wasn't any default backend for Rigetti, so I made RigettiSimulatorBackend default.