Introduce observable key for Estimator

[ikkoham]

Summary

Fixes #8836

Details and comments

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[coveralls]

Pull Request Test Coverage Report for Build 3186947918

• 16 of 16 (100.0%) changed or added relevant lines in 4 files are covered.
• 125 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.2%) to 84.55%

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Files with Coverage Reduction	New Missed Lines	%
src/stochastic_swap.rs	3	87.04%
src/dense_layout.rs	4	91.89%
src/sampled_exp_val.rs	14	63.93%
src/results/marginalization.rs	47	61.49%
src/optimize_1q_gates.rs	57	4.84%

Totals
Change from base Build 3184782875:	-0.2%
Covered Lines:	61556
Relevant Lines:	72804

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💛 - Coveralls

[mrossinek]

Thanks, @ikkoham, for the quick fix of this!

I'm approving this here to point out that this also appears to fix the issues I was observing with qEOM over in qiskit-community/qiskit-nature#856 which triggered opening #8836