[mlir][linalg] Add getIteratorTypesArray to LinalgInterface.

Summary:
Most of the code that gets `iterator_types` from LinalgInterface is forced to
extract values from an `Attribute`. As a result, the usage pattern looks like
this:

```
SmallVector<StringRef> iterators = llvm::to_vector<4>(linalgOp.iterator_types().getAsValueRange<StringAttr>());
```

It also forces all operations that implement LinalgOp interface to have
`iterator_types` attribute even when the information can be easily infered from
other parameters.

In perfect future, `getIteratorTypeArray` should be the only method to get
iterator types from the interface. The default implementation can rely on
`iterator_types` attribute though.

The name `getIteratorTypeArray` was picked to be consistent with existing
`getIndexingMapsArray`.

This patch add a few sample usages. More cleanups will follow.

Differential Revision: https://reviews.llvm.org/D134729