[DWRF] ORC filetype check in ReaderImpl instead of OrcInputFormat

Summary: This is done beause there can be clients directly creating ReaderImpl instead of OrcInputFormat. Having all the checks done in ReaderImpl would make it tackle for both code paths.
Also, made the check function a public static method so that clients can make use of it if needed

Test Plan: Ran filedump utility
Merge pull request #19 from tejasapatil/D1753505

[DWRF] ORC filetype check in ReaderImpl instead of OrcInputFormat