Add BackendEstimatorV2 interface for issue #145

[rosspeili]

Summary

Adds a C++ Estimator V2 interface parallel to the existing sampler stack, addressing #145.

• EstimatorPub, ISA circuit + Pauli observables ((label, coefficient) pairs), with to_json()
• EstimatorPubResult / EstimatorResult, expectation value and standard deviation per pub
• BackendEstimatorV2::run() → BackendEstimatorJob (same pattern as BackendSamplerJob)
• Estimation via basis rotations + BackendSamplerV2 (client-side, no native estimator backend API yet)
• samples/estimator_test.cpp (Bell state + ZZ / XI), CMake target, reno note, README mention

Closes #145

Details and comments

• Mirrors sampler file layout and IBM license headers as in other files, and does not extend BasePrimitiveJob (estimator pubs differ from sampler pubs).
• Skips adding measurements if the ISA circuit already has them.
• estimator_test requires the same runtime setup as sampler_test (QISKIT_IBM_RUNTIME_C_ROOT / QRMI / SQC + credentials).

Testing: Setting up the local Qiskit C build to run test/ and compile estimator_test, but it takes some time on my machine, will also rely on CI on this PR for the standard test_circuit / test_transpiler / test_parameter suite (same bar as upstream). No new test/ cases, just sample integration.

• I have read the CONTRIBUTING document
• Release note: releasenotes/notes/add-backend-estimator-v2-145.yaml
• Sample program added (estimator_test.cpp)
• README updated for estimator build/run
• Unit tests under test/

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