Add expressions support for Virtual Tables

[benbellick]

Currently, VirtualTables can only handle Literals. However, VirtualTables can actually handle Expressions as well, and Literals are being deprecated:

  // A table composed of expressions.
  message VirtualTable {
    repeated Expression.Literal.Struct values = 1 [deprecated = true];
    repeated Expression.Nested.Struct expressions = 2;
  }

from here.

Substrait-java should be able to handle Expressions.