## Description

This PR adds a parameter to configure the input decorator hint fade transition duration.

This animation is not part of the Material specification.
Removing it was considered but it breaks internal tests (see https://github.com/flutter/flutter/pull/107406).
I also considered several ways to avoid the fade animation (setting duration to 0, removing the hint text, etc) but it breaks many existing tests that assumes the hint text to be visible.

To mitigate the issue in a non disruptive way, I set the default duration to 20ms (an arbitrary short value).

## Related Issue

Fixes https://github.com/flutter/flutter/issues/20283.

## Tests

Adds 3 tests, updates 3 tests.