Proof of concept of adding a Feed type

[habrahamsson-skanetrafiken]

Summary

This is a proof-of-concept of how #6515 could be implemented. It is not a complete solution and won't compile. It's just meant to illustrate the proposal.

[t2gran]

A Feed i OTP is set of consistent data. In this PR thare is a Feed that is only about creating IDs. This is probably a FeedScopedIdFactory.

So, I suggest the following:

interface OtpIdFactory {
  FeedScopedID createId(String entityId);
}

class FeedScopedIdFactory  implement OtpIdFactory {
  FeedScopedID createId(String entityId);
}

The createId(..) retuns FeedScopedID for now - since it is the only ID we support, but we can shange this in the future to OtpId and lett FeedScopedID and NetexIdinherit from these. This will support mixing GTFS and Netex feeds without the hack we do to adjust the NetexIds today.

Future improvements
This is not directly handled by this PR, by we should move toward this goal.

The usecase we should aim to support is:

A Feed may have a IDFactory creating IDs specific to the feed (GTFS or Netex). Serializing ID in the API is simple, but input ids need a form of strategy to determin witch Feed it matches.