Make SelectPolygonVolume.CropInPolygon return indices

[jamesdi1993]

Following issue created at https://github.com/isl-org/Open3D/issues/3713, in some circumstance it would be really great to have the indices of the cropped point clouds in SelectionVolume.CropInPolygon() function, for example, when the point cloud size is very large, and it's expensive to make a copy of the cropped point clouds.

This PR proposes to make the SelectionVolume.CropInPolygon() a public method, which directly returns the indices for the cropped point cloud. Python bindings function is added to make it callable from python.

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[update-docs[bot]]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[yuecideng]

Hi, thanks for this PR. Please see style guide and apply it to this PR @jamesdi1993 .