Add UniqueConnIdRule rule and unittest

[edeNFed]

Adds UniqueConnIdRule rule to upgrade/rules as per:

https://github.com/apache/airflow/blob/master/UPDATING.md#unique-conn_id-in-connection-table

Closes: #11037

---

Read the Pull Request Guidelines for more information.
In case of fundamental code change, 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 UPDATING.md.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst)
Here are some useful points:

• Pay attention to the quality of your code (flake8, pylint and type annotations). Our pre-commits will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example DAG that shows how users should use it.
• Consider using Breeze environment for testing locally, it’s a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: dev@airflow.apache.org
  Slack: https://s.apache.org/airflow-slack

[dimberman]

@edeNFed please get tests to pass and then I'll merge

[turbaszek]

@edeNFed please get tests to pass and then I'll merge

@dimberman do you have an idea how to do this? I can see that tests were skipped in every PR opened to v1-10-test...
#11132
#11185

[potiuk]

@edeNFed Can you please rebase to latest v1-10-test ? It should work now.

[edeNFed]

@dimberman @turbaszek all tests passed

[turbaszek]

After the suggested change it looks good to me:

Connection.conn_id is not unique
--------------------------------
The `id` column in the `connection` table must be unique. Previously, this rule was enforced by application logic, but was not enforced by the database schema.

If you made any modifications to the table directly, make sure you don't have duplicate values in conn_id column.


Problems:
  1.  Connection.conn_id=aaa is not unique.

[github-actions]

The CI and PROD Docker Images for the build are prepared in a separate "Build Image" workflow,
that you will not see in the list of checks (you will see "Wait for images" jobs instead).

You can checks the status of those images in The workflow run

[boring-cyborg]

Awesome work, congrats on your first merged pull request!