Update test connection functionality to use custom form fields

[josh-fell]

Closes: #21188

Currently the test connection functionality in the UI doesn't account for custom form fields. This PR intends to include custom form fields as part of the testing mechanism.

Before	After

When testing, if the Extra field is not a valid JSON, an error message is displayed to the user to help provide some direction. (The simple JSON parsing check doesn't definitively assess whether the value is valid JSON but the edge cases like true, {false}, 1, etc. would be effectively useless for the connection anyway. Thinking providing some feedback to users was valuable here especially for syntax errors of manually entering a JSON object.)

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[potiuk]

Thanks @josh-fell for brave attempt to improve that piece of javascript ;)

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.

[potiuk]

Just the docker-compose timout problem

[jedcunningham]

I'm going to kick this to 2.3.0 because of the number of other changes that have happened to connection testing since 2.2.x forked off. We can always reevaluate for 2.2.5 (if there is one) if we think this would work on its own.