inject license info into asset metadata on save/publish

currently license is only at the dandiset level. as part of our metadata injection process (for non-published assets), we should add the dandiset license into the asset metadata. to prevent doing a migration again, we can check for this during publish and inject the license info.

published assets license cannot change.

presently the schema does not require this for an asset, but i think we should do this for appropriate licensing (especially for future use in other dandisets).