[flutter_adaptive_scaffold] allow users to override navigation rail padding from AdaptiveScaffold constructor

[jmiller-pwi]

1. Moved the default padding value into the body of AdaptiveScaffold.standardNavigationRail, and made the padding property an optional nullable value.
2. Added an optional nullable EdgeInsetsGeometry parameter AdaptiveScaffold.navigationRailPadding to the constructor, which is passed into the navigation rail in build.

Fixes issue #161670

[flutter-dashboard]

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