Explorer doesn't know how to "sample" xarray objects

When the data sent to the explorer is a large xarray object and options `rasterize` and `datashader` are turned off, the explorer tries to sample the data to reduce its size. However, it only knows how to do so with pandas DataFrames : `df.sample(...)`.

I'm not sure how one would do that "sampling" with xarray objects, but maybe it could raise a clearer error when encountering a xarray `DataArray` ?

https://github.com/holoviz/hvplot/blob/58ecac37fed12104f5192b77c05fa0d046d57058/hvplot/ui.py#L697