Feat: Add support for European Sleeper

[MartinLangbecker]

This PR adds European Sleeper (ES) as another train operator from which connections can be fetched. All additional connections will be added to the same CSV file as NightJet connections.

Features

• added call to API endpoint for European Sleeper
• added train connection ES 452/453 Praha - Bruxelles Midi
• reused and unified some data structures and services
• all features that worked for NightJet should work for European Sleeper as well

Notes

• station IDs for European Sleeper differ from HAFAS IDs and can only be retrieved from https://www.europeansleeper.eu
• for ES API reference, see https://github.com/MartinLangbecker/night-train-apis/blob/main/european-sleeper-api.yaml (WIP)