Remove the public field accessor of FileScanTask #3130

Is your feature request related to a problem or challenge?

FileScanTask should always be valid, current builder doesn't verify it when builds, and the pub field modifier may make it invalid.

Describe the solution you'd like

1. Verify the result when building
2. Remove pub field accessor, and adds pub getters.

Willingness to contribute

I can contribute to this feature independently