[AIRFLOW-5843] Add conf option to Add DAG Run view

[JCoder01]

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Issue link: AIRFLOW-5843

Adds input field to allow passing of conf when adding a dag run in the UI.

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[ashb]

This has no tests, and there should also be a check to ensure that the conf field is valid JSON.

[JCoder01]

@ashb is there a preferred way to validate the JSON or should I just add a try except with something like "Conf JSON isn't valid"?

[ashb]

@JCoder01 Yeah a try/except is probably good enough.

[JCoder01]

@ashb Ok, can you have a look?

[ashb]

Other than the exception message this looks good.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6287f81). Click here to learn what that means.
The diff coverage is 52.94%.

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  Coverage          ?   32.33%           
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  Files             ?      863           
  Lines             ?    40458           
  Branches          ?        0           
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Impacted Files	Coverage Δ
airflow/www/views.py	25.53% <100%> (ø)
airflow/www/validators.py	29.62% <45.45%> (ø)
airflow/www/forms.py	92% <60%> (ø)

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[kaxil]

Good work @JCoder01