[assets] Granular org fallback

As of right now, the org fallback only works if you have no site config. We should inherit on a row basis so that if a site has a single property (`aem.assets.link.type`) the rest still falls back to org so you don't have to duplicate your repoId, etc.

Work is in da-assets.js. Good first issue.

Start by reading da-assets.js, where the org and site configuration fallback is implemented. Confirm that a site-level aem.assets.link.type overrides only that property while remaining properties, such as repoId, continue inheriting from the org configuration.

Written by the indexing model from the issue text.