[mlir] Introduce DefaultValuedOptionalAttr

Currently DefaultValuedAttr is confusingly actually default valued &
optional but that was an artifact of development and longstanding TODO
to address. Add new attribute that matches this behavior for cases where
that is actually the desired behavior before addressing TODO (e.g., this
is an incremental step to fixing DefaultValuedAttr).

Differential Revision: https://reviews.llvm.org/D130679