Add backend related methods to QiskitRuntimeLocalService

[jyu00]

What is the expected feature or enhancement?

QiskitRuntimeLocalService was created to support local testing mode and is meant to be similar to QiskitRuntimeService. The initial release of QiskitRuntimeLocalService only has the run() method, and we immediately got feedback asking for the other methods 🙂.

This issue is about adding backend related methods to QiskitRuntimeLocalService.

Acceptance criteria

Add the following methods to QiskitRuntimeLocalService

• backends
• backend
• least_busy

In local testing mode, the only backend accessible are the fake backends, hence these methods should only return fake backends (e.g. backend("fake_sherbrooke") would work but not backend("ibm_sherbrooke"))