Expose more downsample algorithms and options

[maximlt]

At the moment, I think, downsample=True is the only way to toggle the downsample1d and there's no way to customize how it's called, i.e. there's no way to set algorithm or some of its other options. Multiple options to implement that:

• Overload downsample to accept a string (minmax) or a dict ({'algorithm': 'minmax', **other_downsample1d_opts})
• Just allow extra kwargs to be passed to hvPlot and catch them internally, like max_px and threshold for dynspread.
• Probably many other ways

Small preference for the first option.

cc @droumis

[droumis]

IMO: overload downsample to accept a string. True could still default to use LTTB but we should allow for others, like downsample='minmax', etc