Improve the provider selection.

[aabizri]

The problem

Currently we implement provider selection through the URL only. However this has the downside that for some providers that restrict some endpoints (like SNCF), we can't stop a request before executing it.

The solution

One way we could do this is move the providers info from URLs to a proper Struct, with information about which endpoints are allowed/disallowed, which will be integrated in Session via a map[string]bool (has O(1) complexity) allowing us not to waste bandwidth.

This may also allow us to have the SSL fingerprints public in those structs, though that's a question for another issue.

As always, I'm open to suggestions.