Create a `after_server_startup` extension point for hooks between startup and first requests

[lehnerchristian]

Currently we have several extension points which are loaded on server startup, however due to their lazy loaded nature they're only loaded and executed when the first requests arrive.

this may be too late for example for in-memory caches, which should already be filled when the first requests arrive

with a new extension point, which is loaded after the plugins are have been loaded