[mlir][tensors] Introduce attribute interface/attribute for tensor encoding

The new "encoding" field in tensor types so far had no meaning. This revision introduces:

1. an encoding attribute interface in IR: for verification between tensors and encodings in general
2. an attribute in Tensor dialect; #tensor.sparse<dict> + concrete sparse tensors API

Active discussion:
https://llvm.discourse.group/t/rfc-introduce-a-sparse-tensor-type-to-core-mlir/2944/

Reviewed By: silvas, penpornk, bixia

Differential Revision: https://reviews.llvm.org/D101008