Support session

[merav-aharoni]

Summary

Adding support for Session in qiskit-ibm-provider.

Details and comments

A _session member was added to IBMBackend.
This update introduces duplications between this repo and qiskit-ibm-runtime. They should be addressed after this one is merged. See related issue: Qiskit/qiskit-ibm-runtime#969.
Fixes #588.

[merav-aharoni]

We also need to update the relevant tutorial. I prefer to do this in a separate PR, so that we first finalize the implementation details here.

[coveralls]

Pull Request Test Coverage Report for Build 5941017993

• 64 of 94 (68.09%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 51.349%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit_ibm_provider/api/clients/runtime.py	1	2	50.0%
qiskit_ibm_provider/api/rest/runtime.py	2	3	66.67%
qiskit_ibm_provider/api/rest/runtime_session.py	8	11	72.73%
qiskit_ibm_provider/utils/converters.py	9	14	64.29%
qiskit_ibm_provider/session.py	23	32	71.88%
qiskit_ibm_provider/ibm_backend.py	19	30	63.33%

Totals
Change from base Build 5940147572:	0.2%
Covered Lines:	3368
Relevant Lines:	6559

---

💛 - Coveralls

[merav-aharoni]

I made a few additional changes since your last review:

1. Added/modified checks inside the tests following your comment above.
2. Added handling of session_id in IBM_Job. I noticed this was missing while doing (1).
3. Added the new test file to integration-test-3 in the Makefile.

I think this will require an additional review.