[mlir][ods] Generalize default/optional parameters

This patch consolidates the notions of an optional parameter and a
default parameter. An optional parameter is a parameter equal to its
default value, which for a "purely optional" parameter is its "null"
value.

This allows the existing `comparator` and `defaultValue` fields to be
used enabled more complex "optional" parameters, such as empty arrays.

Depends on D133812

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D133816