[New Feature]: Use a prerendered image for the layer legend

[jtroberts]

Checked for duplicates

Yes - I've already checked

Alternatives considered

Yes - and alternatives don't suffice

Related problems

I have an external WMS/WMTS service that provide prerendered legends for its image layers and would like to use them in MMGIS.

Describe the feature request

I want an option to specify an external legend in the configuration page and have it display for the layer legend.