Specify service parameter channel through env variable

[kt474]

Summary

channel can now be stored as an environment variable so the service does not have to be instantiated manually and instead can be created from the env variables.

Example:

from qiskit_ibm_runtime import Sampler
import os

os.environ["QISKIT_IBM_CHANNEL"] = channel
os.environ["QISKIT_IBM_TOKEN"] = token
os.environ["QISKIT_IBM_INSTANCE"] = instance
os.environ["QISKIT_IBM_URL"] = url

with Sampler(...) as sampler:
    result = sampler(...)

Details and comments

Fixes #444

[coveralls]

Pull Request Test Coverage Report for Build 2798709559

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 62.635%

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Totals
Change from base Build 2798688893:	0.2%
Covered Lines:	2895
Relevant Lines:	4622

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💛 - Coveralls