[mlir][python] Expose `PyInsertionPoint`'s reference operation (#69082)

The reason I want this is that I am writing my own Python bindings and
would like to use the insertion point from
`PyThreadContextEntry::getDefaultInsertionPoint()` to call C++ functions
that take an `OpBuilder` (I don't need to expose it in Python but it
also seems appropriate). AFAICT, there is currently no way to translate
a `PyInsertionPoint` into an `OpBuilder` because the operation is
inaccessible.