Add radio button rendering for enum Dag Params

[fat-catTW]

Why

Dag authors currently cannot choose a radio button UI for enum Params in the Trigger Dag form. This makes small, fixed choice sets less convenient, especially when the choices are generated from deployment-local configuration such as an INI file.

Solution

Add an opt-in x-airflow-ui.widget = "radio" schema hint for enum Params. Enum Params continue to render as dropdowns by default, while Params with the radio widget hint render as radio buttons.

The Params documentation and tutorial Dag also show how to generate enum choices from a colocated INI file at Dag parse time, so the Trigger Dag form uses serialized Dag Params instead of calling a dynamic choice provider.

Helps address #56632.

Screenshot

Trigger Dag form showing an enum Param rendered as radio buttons:

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[bbovenzi]

I'm wary of adding a new x-airflow-ui option to declaring params. First the name isn't great. Second, we should define how we plan to expand this in the future.

Second, I am not sure if this closes the old issue because the person asked for a selector or a radio over plain text. I don't think it matters as much which of the two exist.

Third, the radio has no option to unselect when it is an optional param.