Allow templates for complications colours

[TomBrien]

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

Currently we define text and gauge colours when creating a complication and the colours are static from that point on. It would be a nice improvement to allow templates for the colours.

Describe the solution you'd like

Keep the current colour picker to allow a static HTML (or other spec) colour to be picked. Store this as a string but allow user to enter text in the field. Then when the complication is updated, if string begins #, hsl( etc. use colour, if it begins {{ render template. I guess we would use template sensors on HA side to generate colour as HTML, HSL or whatever.

Additional context

Also ties into the newish colour picker in HA, bonus points if we can get gradients like watchOS weather and AQ gauges

[bcmitri]

It looks like the gradient request is already covered in #1163.