[mlir][Python][Linalg] Add support for captures in body builder.

When Linalg named ops support was added, captures were omitted
from the body builder. This revision adds support for captures
which allows us to write FillOp in a more idiomatic fashion using
the _linalg_ops_ext mixin support.

This raises an issue in the generation of `_linalg_ops_gen.py` where
```
  @property
  def result(self):
    return self.operation.results[0] if len(self.operation.results) > 1 else None
```.
The condition should be `== 1`.

This will be fixed in a separate commit.

Differential Revision: https://reviews.llvm.org/D100363