Make endpoints in requests optional

[Kaliumhexacyanoferrat]

As adapters can probably not provide endpoint information, they should be optional, so the logic within the handlers has to decide what has to happen then (instead of failing to serve due to an exception being thrown).

[Kaliumhexacyanoferrat]

Main use case for this is to check for HTTPS to automatically generate valid URLs (e.g. for redirects or OpenAPI scenarios). There is no good fallback for those modules (as they would generate invalid URLs), so we probably should find a way for the adapter engines.