Move class OpenApiAccess to package exasol/saas/client

[ckunki]

Current Situation

Currently, this repo already contains some initial integrations tests verifying the API itself based on

• some small private fixtures in file integration/conftest.py
• which in turn are based on class _OpenApiAccess in file api_access.py which is a private class in a file, that is part of the test code not published to pypi and not accessible by dependent projects.

The current ticket requests to

Move file api_access.py to production tree at exasol/saas/client/api_access.py

`* Include it into publications on pypi

• Make it accessible for dependent projects, esp. pytest-plugins
• Make class _OpenApiAccess public by renaming it to OpenApiAccess