[mlir][ODS] Add support for optional operands and results with a new Optional directive.

Summary: This revision adds support for specifying operands or results as "optional". This is a special case of variadic where the number of elements is either 0 or 1. Operands and results of this kind will have accessors generated using Value instead of the range types, making it more natural to interface with.

Differential Revision: https://reviews.llvm.org/D77863