Provide a plugin mechanism to set the logo image

[rakillen]

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

In our application, we would like to replace the logo image at the top of the screen and on the slider menu, using pkg plugin configuration. We currently have two approaches:

• Create a directory under shell/assets/brand with our logo images, and edit the ui-brand setting to match the directory name. This sets the images correctly, but requires us to add files outside the plugin directory.
• Directly edit the ui-logo-dark and ui-logo-light settings to install our logo images. This requires some extra installation steps to encode and update those settings.

Describe the solution you'd like

We'd like to have our logo images in the plugin directory structure, maybe under brand. Possibly the BrandImage.vue code could check the plugin's subdirectory as part of its search. I don't think it would need to take the ui-brand setting into account for the plugin directory.

Describe alternatives you've considered

The isSingleProduct configuration used by the epinio plugin can specify a logo image, but has some limitations that won't work for our application:

• The slider menu TopLevelMenu is hidden when there is a single cluster with this option. Our users need access to the menu options, so we would have to recreate those links on our landing page.
• The slider menu is available when there are multiple clusters, but the menu icon overlaps the logo icon visually.
• The setIsSingleProduct option logoRoute doesn't have an alternative for dark mode.

Additional context

[robertpatrick]

@richard-cox any thoughts on this? Are we missing something obvious?