[WIP] Extend Optional op

[gramalingam]

Description

This PR extends the existing Optional by adding attributes (similar to the Constant op) to specify values via an attribute instead of an input.

Motivation and Context

This is intended to support authoring function-ops with optional attributes, particularly when the behavior of the function when the attribute value is not specified cannot be captured using a default-value for the attribute.

This should allow us to author functions as below, where alpha is an optional attribute. The first-line promotes the optional-attribute into a dynamic-optional-value. This allows us to process the dynamic-optional-value using a conditional (If) op.

my_function <alpha> (X) => (Y) {
   alpha_opt = Optional < value : int = @alpha, type = int64 >()
   cond = OptionalHasElement(alpha_opt)
   Y = If (cond) ...
}