DRILL-6084: Show Drill functions in WebUI for autocomplete
Building on top of DRILL-3988 and leveraging DRILL-5868, this allows support for Drill functions to be now available in the WebUI.
If users wants UDFs to show up, they should place the UDF jars in the `$DRILL_HOME/jars/3rdparty` directory so that this can be loaded during the Drillbit's startup.
Concept of internal Drill functions are introduced. With this, internal Drill functions like `ConvertToNullableXYZ` has been marked as internal.
The WebUI will not show these functions. However, they are still visible in `sys.functions` table with an additional column indicating that it is an internal function.
Tests have been added as a part of this commit to verify the internal functions concept.
