[mlir][transform][bufferization][python] Add mix-in classes for two ops.

This patch adds mix-in classes for the Python bindings of
`EmptyTensorToAllocTensorOp` and `OneShotBufferizeOp`. For both classes,
the mix-in add overloads to the `__init__` functions that allow to
construct them without providing the return type, which is defaulted to
the only allowed type and `AnyOpType`, respectively.

Note that the mix-in do not expose the
`function_boundary_type_conversion` attribute. The attribute has a
custom type from the bufferization dialect that is currently not exposed
in the Python bindings. Handling of that attribute can be added easily
to the mix-in class when the need arises.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D155799