Remove Dask as a hard dependency of Datashader

[Azaya89]

Is your feature request related to a problem? Please describe.

When using Datashader (either directly or interactively as a parameter in hvPlot) on a project that doesn't use Dask, you get a future warning of the form:

FutureWarning: Dask dataframe query planning is disabled because dask-expr is not installed.

You can install it with `pip install dask[dataframe]` or `conda install dask`.
This will raise in a future version.

  warnings.warn(msg, FutureWarning)

However, this warning is removed when Dask is added as a dependency of the project

Describe the solution you'd like

It will be nice to NOT have dask as a dependency for using datashader, especially when the specific case doesn't require the use of Dask.

Describe alternatives you've considered

I have temporarily added Dask as a dependency for the current examples I've encountered it

Additional context

Example screenshot - Using rasterize in hvPlot:

[jbednar]

Thanks! Right now dask-core is a hard dependency, and dask-expr (brought in by installing dask) turns out to be a soft dependency. Ideally, as in #1350, we won't have any dependency at all, so that users not installing dask will never see anything to do with Dask to confuse them. Hope #1350 works well!