[mlir][transform] Add support for expressing scalable tile sizes

This patch enables specifying scalable tile sizes when using the
Transform dialect to drive tiling, e.g.:
```
%1, %loop = transform.structured.tile %0 [[4]]
```
This is implemented by extending the TileOp with a dedicated attribute
for "scalability" and by updating various parsing hooks. At the moment,
only the trailing tile size can be scalable. The following is not yet
supported:
```
%1, %loop = transform.structured.tile %0 [[4], [4]]
```

This change is a part of larger effort to enable scalable vectorisation
in Linalg. See this RFC for more context:
  * https://discourse.llvm.org/t/rfc-scalable-vectorisation-in-linalg/

Differential Revision: https://reviews.llvm.org/D150944