Implement direct-access API

[airwoodix]

Summary

This PR implements the direct-access API. This API allows evaluating quantum circuits on AQT devices by communicating with the device directly.

The AQTProvider.get_direct_access_backend() method retrieves a Qiskit backend using the direct-access API. The returned object is mostly compatible with the instances targeting cloud resources returned by AQTProvider.get_backend().

The API reference in the documentation was modified to include the direct-access APIs and datastructures. Adding those to the user guide should be done in a later step, once the use case stabilizes.

Details and comments

Compatibility and differences between AQTResource and AQTDirectAccessResource

• both classes implement Qiskit's BackendV2 interface.
• the job types returned by their run() method differ (AQTJob vs AQTDirectAccessJob). Both job types implement Qiskit's JobV1.
• the semantics of submit() and result() in the job types differ. For AQTJob, the circuit batch is submitted when calling submit(). For AQTDirectAccessJob, the job batch is iteratively submitted when calling result().
• the compatibility of both backend types with typical Qiskit usage is tested in test_execution, where most test cases are run with both backend types.

Offline simulators

Since there would (should!) be no behavior difference between a direct-access offline simulator and a cloud-like one, only the latter (historically first) kind is provided. The former is only available in tests, to ensure the common behavior.

Authentication

The provider forwards its token on the direct-access API, whether used or not.