Allow plugins to specify a custom icon

[scothis]

Describe the problem/challenge you have

Clarity has a nice generic set of computing related icons, but the odds of Clarity having an icon appropriate for a plugin, which are often tied to a specific project, is unlikely.

Describe the solution you'd like

Plugins should be able to define a custom navigation icon. For example, a Knative plugin should use the Knative logo, and not a generic cloud icon.

Anything else you would like to add:

Data URIs are a good way to avoid plugins needing to reference external assets. To aid theming and visual consistency, a single SVG stroke could be allowed that is then styled by the client to match the current theme.

[ftovaro]

I can take a look at this one, If I require more info I'll be posting it here