Support multiline input for Params of type string in trigger UI form

[sc-anssi]

---

This patch adds the possibility to have a multiline input (textarea) for Params of type "string" in the trigger UI form. The choice between rendering a text input (single line) or a textarea (multiline) is based on the presence of a new line character in any of the examples of the Param. For instance, the following code generates a multiline input:

Param(
    description="a parameter that is rendered better with a textarea",
    type=["string", "null"],
    examples=["first line of text before\na new line"],
)

I know Params of type "array" are rendered as textareas but forcing our end-users to format their multiline text into an array of strings would not be very friendly :). So it seems more practical to leave the Param as a "string" type and work on the rendering options instead.

The patch is merely a PoC to showcase what the feature would look like and by no means a definitive solution. I think that relying on the examples attribute is more a hack than a solution, but adding a new explicit attribute to the Param object is beyond my understanding of Airflow. So feel free to rewrite it completely if you think the feature is worth it.

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