Ensure that getAvailablePaths respects the maxDepth of the file iterator

[jaxwilko]

This PR fixes an issue brought to light by wintercms/winter#791.

When FileDatasource::getAvailablePaths() is called, it does not respect the same maxDepth setting that is set in FileDatasource::select() meaning that the reported paths differ from the selectable paths.

This PR adds the setMaxDepth() call on the iterator used in getAvailablePaths() and sets a property for maxDepth to remove magic numbers and allow for custom extensions.

[what-the-diff]

• Added a new property to the class: $maxDepth
• Changed setMaxDepth() from 1 to use max depth instead of hardcoded value
• Updated getAvailablePaths() method so that it uses max depth +1 for RecursiveIteratorIterator::setMaxDepth(). This is because we want all subdirectories, not just one level down (which would be 0).