Allow scrollPadding to be defined in InputDecorationTheme

[MilesAdamson]

Use case

I would like 40 padding on text field scrolling (opposed to the default 20) on every text field in my app.

Currently you must supply this to every text field individually.

          const TextField(
            scrollPadding: EdgeInsets.all(40),
          ),
          const TextField(
            scrollPadding: EdgeInsets.all(40),
            decoration: InputDecoration(
              labelText: "This is a label",
            ),
          ),
          const TextField(
            scrollPadding: EdgeInsets.all(40),
            decoration: InputDecoration(
              hintText: "This is a hint",
            ),
          ),
          const TextField(
            scrollPadding: EdgeInsets.all(40),
            decoration: InputDecoration(
              helperText: "This is helper text",
            ),
          ),

Proposal

InputDecorationTheme(
    contentPadding: const EdgeInsets.all(16.0),
    // add this just like contentPadding
    scrollPadding: const EdgeInsets.all(40.0),
  );

[flutter-triage-bot]

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