Add support for viewing backends available on IBM Cloud

[rathishcholarajan]

Summary

This PR builds upon #55 (will rebase this once #55 is merged) to make the following functionality available to IBM Cloud users. (Currently works only with staging env)

• backends()
• get_backend()
• backend.status()
• backend.properties()
• backend.defaults()
• backend.configuration()

Example:

from qiskit_ibm_runtime import IBMRuntimeService
service = IBMRuntimeService(auth="cloud", token="<staging_api_key>", locator="<staging_locator>")

# List backends
service.backends()

# Get a backend by name
simulator = service.get_backend("ibmq_qasm_simulator")

# Get a backend's status
simulator.status()

# Get a backend's properties
simulator.properties()

# Get a backend's configuration
simulator.configuration()

# Get a backend's pulse defaults
simulator.defaults()

Details and comments

Fixes #