refactor!: Seed Finders create collection of `Seed<external_spacepoint_t>` instead of `Seed<Proxy<external_spacepoint_t>>`

[CarloVarni]

As of not the Acts::SpacePointContainer object stores a collection of proxies internally. This was necessary due to the fact that seed finders runs on the proxies and return a collection of seeds of proxies. Since the seeds do not take ownership of the proxies (they have pointers to them), the proxies have to remain alive. That was not the case for the orthogonal.

This PR changes the behaviour. Now the Acts::SpacePointContainer object does not store proxies.

In order to make the code work, the seed finders have been modified so that they do not return a collection of seed of proxies, but a collection of seed of external space points.