Allow users to indicate they are done submitting jobs to a session

[kt474]

Summary

WIP: PR is branched from #1119, so ideally that is merged first

• Re-purposed session.close() to indicate one is done submitting to the session by using accepting_jobs = false.
• New method session.cancel() terminates all jobs inside a session (this is the old close method, still calls /close).
• session.close() is called upon exiting session in context manager

Details and comments

Fixes #1008

[coveralls]

Pull Request Test Coverage Report for Build 6451540490

• 28 of 39 (71.79%) changed or added relevant lines in 5 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.09%) to 74.692%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit_ibm_runtime/api/clients/runtime.py	2	3	66.67%
qiskit_ibm_runtime/api/rest/base.py	10	14	71.43%
qiskit_ibm_runtime/api/rest/runtime_session.py	9	15	60.0%

Files with Coverage Reduction	New Missed Lines	%
qiskit_ibm_runtime/api/clients/runtime.py	1	54.79%
qiskit_ibm_runtime/session.py	1	98.81%

Totals
Change from base Build 6397643118:	-0.09%
Covered Lines:	2547
Relevant Lines:	3410

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💛 - Coveralls

[kt474]

@beckykd should docs updates be in a separate PR? I'll probably need help with the wording on a lot of these - it will also be a bit tricky since once this is released, older versions of qiskit-ibm-runtime will behave differently

[kt474]

closing this and reopening on a fresh branch since this no longer depends on a different PR