[mlir][sparse] Improving SparseTensorDimSliceAttr methods

This patch makes the following changes to `SparseTensorDimSliceAttr` methods:
* Mark `isDynamic` constexpr.
* Add new helpers `getStatic` and `getStaticString` to avoid repetition.
* Moved the definitions for `getStatic{Offset,Stride,Size}` and `isCompletelyDynamic` out of the class declaration; because there's no benefit to inlining them.
* Changed `parse` to use `kDynamic` rather than literals.
* Changed `verify` to use the `isDynamic` helper.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D150919