Improvement: Added JSON linter to connection edit / add UI

[maggesssss]

I have a proposal for an improvement on the connection edit / add UI.

conn.extra only allows valid JSON.
On the UI, the widget is a simple TextArea without linting capabilities.

I have added the CodeMirror as already used by trigger dag view also on the connection edit / add UI.
This provides a better visual feedback.

Additionally when editing a connection, JSON data of conn.extra is displayed idented instead of a single line:

This is currently just a proposal and I would like to hear some feedback.

[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/main/CONTRIBUTING.rst)
Here are some useful points:

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[boring-cyborg]

Awesome work, congrats on your first merged pull request!