Add ability to provide proxy for dbt Cloud connection

[b-per]

Adding the ability to define proxy settings when connecting to dbt Cloud. This can be useful for people setting up IP restrictions on dbt Cloud, allowing only some IPs to connect.

This is my first contribution to Airflow, so feel free to let me know if I need to do anything else.

I update the code for the hooks and added some documentation.

I didn't add tests as I am not too sure of how I could do it in that case, but I am happy to do so with a bit of pointers/guidance.

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[boring-cyborg]

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[potiuk]

Needs unit tests.

[b-per]

Happy to add some, but I am not able run the existing ones. Hence why I was wondering if there was any pointer or guidance.

If I do a pytest tests/providers/dbt/cloud/ in my hatch env I get a ModuleNotFoundError: No module named 'sqlalchemy' but sqlalchemy is indeed installed

[b-per]

I actually managed to run the tests with breeze testing tests --test-type "Providers[dbt.cloud]".

I have updated them so that they pass again and I have added an extra test with the proxy.

Let me know if instead of creating a new test you prefer if I update all the pytest.mark.parametrize to include the connection with a proxy.

My rationale for adding a new test for the proxy rather than as a parameter for all tests is that most people using the providers won't need the proxy.

[potiuk]

If I do a pytest tests/providers/dbt/cloud/ in my hatch env I get a ModuleNotFoundError: No module named 'sqlalchemy' but sqlalchemy is indeed installed

BTW. Re venv - you should be able to run the test in venv created with `pip install -e ".[dbt.cloud,devel-tests]" - that should install all what is needed to run the tests.

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.