Qiskit / qiskit-ibm-runtime / 30115949862

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Add "physical_qubits" property to backends (#3080)

### Summary

Add `physical_qubits` as a property of `IBMBackend` and (for non-retired
ones) `FakeBackendV2`.

### Details and comments

Fixes #3071

Some forces made the implementation less straightforward than ideal:
* The new property is available at the general `/backends` endpoint
* `IBMBackend` is always instantiated through `QiskitRuntimeService`
* `IBMBackend` design is quite geared towards using the
configuration/properties backend-specific information
* `QiskitRuntimeService` has several layers of state, caching, and
logic, and the point in time where `/backends` is queried is separate
from the point in time where backends are instantiated.

This caused adding a new argument to `IBMBackend` constructor (which I
would like to have avoided), and re-target the
`QiskitRuntimeService._backends_list` into a more generic
`QiskitRuntimeService_backends_info_per_instance` (so it contains all
the information from the `/backends` endpoint for all instances).

Ideally, we should invest time in refactoring `IBMBackend` and
`QiskitRuntimeService` for better modelling of the concepts and
maintainability.

### AI/LLM disclosure

- [x] I didn't use LLM tooling, or only used it privately.

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Co-authored-by: Samuele Ferracin <sam.ferracin@ibm.com>

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