ExtensionManagementMixin generic param bound as STACObject is to restrictive, as Asset and other non-STACObject classes can have extensions applied

[philvarner]

[gadomski]

Assets don't have a stac_extensions attribute, which makes their extension story a bit awkward (see #793 for one example). So I'm not sure ExtensionManagementMixin would be appropriate for an Asset as currently written.