fix: Update custom connection field processing

[mike-mcdonald]

Fixes issue where custom connection fields are not updated because extra field is in form and has previous values, overriding custom field values.
Adds portion of connection form tests to test functionality.

Closes: #20839

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[boring-cyborg]

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[bbovenzi]

lgtm

[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.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!