Expose cost estimation values

[kt474]

Summary

Don't merge until change is live in prod on the server side

Adding a new property, job.usage_estimation which returns

{'quantum_seconds': 3.082247164179738}

Details and comments

fixes #681

[coveralls]

Pull Request Test Coverage Report for Build 5789107852

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 83 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.009%) to 51.151%

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Files with Coverage Reduction	New Missed Lines	%
qiskit_ibm_provider/job/ibm_circuit_job.py	83	48.42%

Totals
Change from base Build 5728346612:	-0.009%
Covered Lines:	3312
Relevant Lines:	6475

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💛 - Coveralls

[merav-aharoni]

Looks good. Only small typo.