Allow for multiple clients in cloud service

[ElePT]

Summary

This PR allows to keep track of all instantiated clients in the runtime service instead of overwriting a single client in the "ibm_cloud" path. The mechanism is quite naive (just storing a dictionary of instantiated clients accessible by CRN), I think that this logic should be handled by the RuntimeClient class instead of the QiskitRuntimeService, but as a first approximation at least this implementation unlocks the following functionality:

• Accessing backends from 2 different instances and then retrieving jobs from the first backend:

service = QiskitRuntimeService(channel="ibm_cloud")
backend_1 = service.backend("ibm_hanoi", instance="CRN_FROM_ACCOUNT_1")
circuit = QuantumCircuit(4,4)
sampler = SamplerV2(backend_1)
job = sampler.run([(circuit)])
job_id = job.job_id()
backend_2 = service.backend("ibm_kingston", instance="CRN_FROM_ACCOUNT_2")
new_job = service.job(job_id)

• Retrieving a session instantiated with a different instance than the one currently used:

service = QiskitRuntimeService(channel="ibm_cloud")
backend_1 = service.backend("ibm_torino", instance="CRN_FROM_ACCOUNT_1")
circuit = QuantumCircuit(4,4)
with Session(backend_1) as session:
        sampler = SamplerV2(session)
        session_id = session.session_id
        job = sampler.run([(circuit)])
        job_id = job.job_id()
backend_2 = service.backend("ibm_kingston", instance="CRN_FROM_ACCOUNT_2")
new_session = Session.from_id(session_id, service=service)

• In general, switching between backends from different instances and always having access to submitted jobs and sessions independently of the instance they were created with.

On top of this, the PR clearly establishes a difference between the ibm_cloud channel and the ibm_quantum_platform channel.

Details and comments

Fixes #