Split-off the API client resources in an isolated sub-package

[airwoodix]

Summary

This patch moves most of the resources associated to the Qiskit-agnostic API client to an isolated sub-package at qiskit_aqt_provider.api_client. The isolation is enforced by the newly configured tach setup.

The realize the isolation, the User-Agent string now declares a different name for the client (i.e. not mentioning qiskit-aqt-provider anymore). The remaining information stays the same.

Follow-up work will include moving all Qiskit-agnostic resources to the qiskit_aqt_provider.api_client package as well as defining a clean public interface for that package (that is, enable tach's strict mode for qiskit_aqt_provider.api_client). This sub-package could then be published as an independent package.

Details and comments

The tach setup also includes running tach check-external. This unfortunately has a number of issues:

• only "main" project dependencies are scanned, so we need to ignore the tests
• extra dependencies are considered unused because none of the extras is relevant for the main code-base. This is in particular the case for the test extra introduced in Add test dependency group #183. Those dependencies need to be manually excluded in the tach configuration.
• running tach check-external as a pre-commit hook breaks because the package metadata is not available for tach to understand that python_dotenv is the distribution name for the dotenv package. This will be fixed upstream.