[mlir][sparse] add ability to select pointer/index storage type

This change gives sparse compiler clients more control over selecting
individual types for the pointers and indices in the sparse storage schemes.
Narrower width obviously results in smaller memory footprints, but the
range should always suffice for the maximum number of entries or index value.

Reviewed By: penpornk

Differential Revision: https://reviews.llvm.org/D92126