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

This patch enables specifying scalable vector sizes when using the
Transform dialect to drive vectorisation, e.g.:
```
transform.structured.masked_vectorize %0 vector_sizes [8, 16, [4]]
```
This is implemented by extending the MaskedVectorizeOp with a dedicated
attribute for "scalability" and by overloading `parseDynamicIndexList`
so that MaskedVectorizeOp can continue using the auto-generated parser
and printer.

At the moment, only the trailing vec size can be scalable. The following
is not yet supported:
```
transform.structured.masked_vectorize %0 vector_sizes [8, [16], [4]]
```

As the vectoriser does not support scalable vectorisation just yet, a
warning is issues when scalable vector sizes are used. You can also use
the debug output, `--debug-only=linalg-vectorization`, to check whether
scalable vectorisation has been switched on.

This change is a part of a larger effort to enable scalable
vectorisation in Linalg. See this RFC for more context:
  * https://discourse.llvm.org/t/rfc-scalable-vectorisation-in-linalg/
Similar patch for tiling: https://reviews.llvm.org/D150944

Differential Revision: https://reviews.llvm.org/D151892