Static-ness is not conserved for asset paths in AssetServer::load()

[cart]

The changes in #10823, specifically the move to impl<'a> From<&'a str> for AssetPath<'a>, make us lose the benefit of CowArc::Static in the common case of asset_server.load("path"). With the new impl<'a> From<&'a str> for AssetPath<'a> approach we're statically erasing the 'static context. This means path.clone() (which we do a number of times in asset_server.load()) now always allocates an Arc<str> internally when we pass in a &'static str.

Given that this is the primary scenario CowArc was built, I believe it is worth fixing. The impl From<&'static str> for AssetPath<'static> was an intentional tradeoff to ensure asset_server.load("path") captures the static lifetime and prevents a clone.