Allow "named" assets and overriding "names"

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

Currently only assets loaded by AssetServer::load have names and it's fixed as the name of the path. Looking up a loaded assets is as simple as calling load a second time. However assets that are created don't have this luxury thus requiring custom cataloging of handles to provide lookup.

## What solution would you like?

Add new methods
```rust
AssetServer::get_handle_by_name(name: &str) -> Option>;
Assets::get_handle_by_name(name: &str) -> Option>;
Assets::add_as(name: &str, asset: T) -> Handle;
```

## What alternative(s) have you considered?

Adding custom catalog for assets. This while doable seems like an anti-pattern as all handles will need to be cloned into the catalog.