[mlir][sparse] introduce a higher-order tensor mapping

This extension to the sparse tensor type system in MLIR
opens up a whole new set of sparse storage schemes, such as
block sparse storage (e.g. BCSR) and ELL (aka jagged diagonals).

This revision merely introduces the type extension and
initial documentation. The actual interpretation of the type
(reading in tensors, lowering to code, etc.) will follow.

Reviewed By: Peiming

Differential Revision: https://reviews.llvm.org/D135206