Virtual catalog columns

Data Explorer has catalog views feature that allows 2 or more catalog tables to be joined together to create a virtual view. The virtual catalog columns extend the catalog views feature by supporting two new capabilities.

1. Ability to add a virtual column to the view that has its contents computed when the query is executed. For e.g., suppose a 2D bounding box is represented by 4 columns(x_left, y_top, x_right, y_bottom) in a catalog table. A virtual column to compute the area of this bounding box can be defined during view creation. On executing the query, the virtual column gets computed from the contents in the 4 columns.
2. Ability to add a virtual column as a type mapping of an existing column. For e.g., suppose catalog table has a column of type double that actually holds prediction confidence from an inference model. Defining a type mapping column allows Data Explorer to
   1. validate that the values of the column are between [0,1].
   2. provide richer functionality, like attaching a confidence score to a bounding box.