Qiskit / qiskit-ibm-runtime / 33101251452

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Improve decoders/ modularization, explicitly avoiding importing from wrapper (#3270)

### Summary

When we started moving (circa #2785) post-processing closer to the
decoders, the utility functions for each wrapper were evaluated,
deciding to move the ones only used by post-processing to
`decoders/...`, keeping the one used for pre-processing in
`executor-x/...`. This was motivated by modularity and prevention of
import cycles, as both modules are heavy and have a tendency of
spreading imports over the codebase.

This PR reinforces such modularity, as it slowly declined as work
progressed, by moving:
* `unbroadcast_index()`
* `get_pauli_basis()`
* the zne extrapolation module (technically only
`process_extrapolated_expectation_values` was used, but in practice
seems to bring the rest of the module)

into `decoders/`. The first two functions are indirectly used by the
wrappers, this change helps in breaking the `decoders` -> `wrappers`
dependency (deemed as more problematic than `wrappers` -> `decoders`).

### Details and comments

Fixes N/A

### AI/LLM disclosure

- [x] I didn't use LLM tooling, or only used it privately.

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