[mlir] Change IteratorType in ContractionOp in Vector dialect from string to enum.

This is the first step in replacing interator_type from strings with enums in Vector and Linalg dialect. This change adds IteratorTypeAttr and uses it in ContractionOp.

To avoid breaking all the tests, print/parse code has conversion between string and enum for now.

There is a shared code in StructuredOpsUtils.h that expects iterator types to be strings. To break this dependancy, this change forks helper function `isParallelIterator` and `isReductionIterator` to utils in both dialects and adds `getIteratorTypeNames()` to support backward compatibility with StructuredGenerator.

In the later changes, I plan to add a similar enum attribute to Linalg.

Differential Revision: https://reviews.llvm.org/D133696