Closes: #19793 - Nav menu link customization

[bctiemann]

Closes: #19793

Changes the PluginMenuItem and PluginMenuButton classes to handle an alternate pre-rendered literal url init parameter, rather than just supporting link which must be a view name suitable for {% url %}.

This allows plugin authors to supply link target URLs that might require additional (arbitrary) kwargs for resolution, rather than relying on built-in NetBox view names.

Note that Django does not provide a supported way of passing arbitrary kwargs to {% url %}; the params must be known in advance and specified explicitly (i.e. {% url 'view_name' arg1='foo' arg2='bar' %}). This prevents us from implementing this functionality in a more elegant way, i.e. being able to pass a link_kwargs dict which could then be plugged into the template directly like {% url item.link **item.link_kwargs %}.

[jeremystretch]

I'm not convinced we need to (or should) support arbitrary executables in place of menu items. Where dynamic iteration is needed, a plugin author can pass an arbitrary object as items within a MenuGroup, and define its __iter__() method to return whatever MenuItems are prudent.

I tested this approach with the custom objects plugin and it works well. The only blocker I ran into there was the rigid requirement for passing a static view name as a MenuItem's link. Rather than introducing a separate, conflicting url parameter maybe it would make sense to introduce a link_kwargs argument that can be passed along with link for URL resolution in the menu item.