Allow to define custom icon uri

[iLLiCiTiT]

Description

Right now we do allow to use only application icons that are available inside applications addon, so custom applications can't set custom icon.

That is due to recent changes in applications addon and movement out of ayon-core.

How would you imagine the implementation of the feature?

Because webactions are comming, we should support custom uri for icon in additional applications. I guess the icons could be supplied by an addon,

Question that might rise is how to make it dynamic enough, the final uri could be like https://my.ayon.server/addons/myAddon/1.1.1/private/icons/myApp.png but the template probably should be {ayon_url}/addons/myAddon/{addon_versions[myAddon]}/private/icons/myApp.png so it can be used dynamically, ayon_url would be filled with whatever client needs and addon_versions would be filled with addon versions based on addons available in bundle that is used.