Variable length Tuple

In https://github.com/holoviz/panel/pull/6912 we are trying to auto-convert Traitlets and Pydantic models to Parameterized models.

When trying to autoconvert Tuples I experience the problem that Params tuples are fixed length while Traitlets and Pydantic are not.

Panel also implements a custom `Margin` parameter to work around this issue.

I've also many times been hit and surpriced by this so many times at work that I've just stopped using `param.Tuple` as my tuples are seldom fixed length.

Please enable `Tuple` to be non-fixed length or provide a `NonFixedLengthTuple` parameter.