Deprecate identifier collection approach.

[dereuromark]

Refs #710

The config has to be only slightly adjusted to be "safer".
Seems BC and easy to switch up.

[markstory]

Will we also support identifier being an object? If we're going to change how authentication is wired up, we could make it more compatible with the DI container.

[dereuromark]

Already done, afaik there is a test case added
https://github.com/cakephp/authentication/pull/712/files#diff-b9a4b24bdb898b49129cd5b64f3b2f3cf9e71da882e8f07dc277d3df0d96394fR143

[dereuromark]

Added also a test for a direct IdentifierCollection passed:
f2c713e
Or is that also deprecated? I guess it can be useful in some rare cases.

[ADmad]

There's AuthenticationService::getIdentificationProvider() which returns the identifier instance which successfully returned the identity. So that method needs to be updated too to get the identifier instance from the successful authenticator (if the IdentifierCollection::getIdentificationProvider() instance for the AuthenticationServices returns null).

[dereuromark]

Good point.
Done :)