Run unit tests with airflow installed from packages

[potiuk]

This PR adds the option of running unit tests for providers against
a specific airflow version (for example released version in PyPI)
and enables it for back-compatibility testing for 2.9.1. In the
future it could be used to run forward-compatibility testing with
Airflow 3 as well.

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^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[potiuk]

As explained in the devlist - This PR - specifically this job https://github.com/apache/airflow/actions/runs/9023444361/job/24795410794?pr=39513 runs Provider unit tests against Airflow 2.9.1 installed from PyPI. I will add a few comments and tag relevant people in the places where I need help to understand if my changes are ok or not as we need to add compatibility layer for our tests to make it works for 2.9.1 (and later we should do the same for 2.8 and 2.7).

[potiuk]

Rebased after some recent additions - still green :). Looking for an approval :)

[vincbeck]

Some nits

[potiuk]

Comments resolved / nits merged :)

[potiuk]

All conversations resolved and answers found :). ... I think we are good to go.

[potiuk]

Finaly :). RIGHT before a meetup where I will talk about CI/CD tests :)