Add rust bin for defining and running custom appviews using pindexer/cometindex

[ejmg]

Summary

Specific AppViews like that suggested in PR #4752 might be better off defined as a binary that is bundled with apps like Cuiloa to prevent them from polluting pindexer with overly specific indexers. Looking at how the web team bundles their web code alongside rust would probably be a decent start for getting something off the ground.