[mlir][linalg] Add an InitTensorOp python builder.

* This has the API I want but I am not thrilled with the implementation. There are various things that could be improved both about the way that Python builders are mapped and the way the Linalg ops are factored to increase code sharing between C++/Python.
* Landing this as-is since it at least makes the InitTensorOp usable with the right API. Will refactor underneath in follow-ons.

Differential Revision: https://reviews.llvm.org/D99000