add layerAssets property to FlightDataPath (#66354)
This adds a new `layerAssets` property (containing styles and script
tags) to `FlightDataPath`. Previously these were lumped in with the
`head` node, but we intentionally only ever render a single `head`, to
avoid duplicating metadata. This would mean `<AppRouter />` would only
ever render imported stylesheets for a single page in a racey way.
However, since Float handles hoisting and deduping these style tags,
we're safe to render them for each segment.
This PR introduces no change in behavior, aside from sending
`layerAssets` down from the server and storing it in the client router
cache. These nodes aren't rendered -- this is done in #66300.
