Legend color option when rasterize is True

#### Is your feature request related to a problem? Please describe.
When using hvplot and setting the rasterize option to True, I lose the ability of having a color next to the label for the item in the legend.

#### Describe the solution you'd like
I would like to be able to provide a color for hvplot to use in the legend. Something similar to how you would set the legend label or axis labels, provide an option to hvplot with a RGB or HEX code for the color to display.

#### Describe alternatives you've considered
I am unaware of any workarounds to solve this issue

Start by reproducing the hvplot case with rasterize=True and inspect how the legend is built for the rasterized plot. Identify the public option for accepting an RGB or HEX legend color, then add regression coverage showing that the supplied color appears beside the label when rasterization is enabled.

Written by the indexing model from the issue text.