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Assets: Store Action sub-type in asset metadata

Blender 3.0 will only support single-frame Actions in the pose library.
The goal of this patch is to lay the groundwork for making it possible
for the Asset Browser to reject/hide "animation snippet" Action assets.

Determining whether an Action has one or more frames (i.e. whether it
has a single pose or animation) requires inspecting the Action itself,
and thus loading the data-block itself. This would make it impossible to
quickly determine from the asset browser.

To solve this, the Action is inspected before saving, and a
`"is_single_frame"` boolean (well, 0/1 integer) IDProperty is added.

Reviewed by: Severin

Differential Revision: https://developer.blender.org/D13202