Add local channel

[jyu00]

What is the expected feature or enhancement?

QiskitRuntimeLocalService was created to support local testing mode and is meant to be similar to QiskitRuntimeService. Today a user is not expected to instantiate this class directly, since it only has a run() method that is called under the cover by the primitives. As we add more methods to it (see #1606 and #1607), users will want to instantiate it. Instead of having to import / create another class, we should just add a "local" channel.

Acceptance criteria

User can do

from qiskit_ibm_runtime import QiskitRuntimeService, Sampler

service = QiskitRuntimeService(channel="local")  # <--- this will return an instance of `QiskitRuntimeLocalService`