Add observers to asset server

## What problem does this solve or what need does it fill?

With procgen I often find I need the asset itself in order to make it usable. E.g. Load a mesh and then add a boolean operator to it. Currently getting asset itself in bevy is very cumbersome.

## What solution would you like?

add `observe` method for `Handle` with common triggers like `OnLoaded`, `OnError`, etc.

## What alternative(s) have you considered?

Listening for asset loader events and check if the resource I want is the one that was just loaded. It's logically similar to the observer method proposed, but requires a lot more boilerplate.

Start by reading the asset loader event approach described in the issue and how Handle currently exposes asset access. Define what an observe method and common triggers such as OnLoaded and OnError would need to cover; done means the proposal supports observing a specific asset without manually filtering loader events.

Written by the indexing model from the issue text.