[WIP] Moving node validation from JL-Doctrine-DBAL to Jackalope

As suggested in the comments for this code in the doctrine-dbal implementation, this PR will aim to move the node validation to here.

The validate method currently also adds additional node operations (e.g. for node autocreate) to the transport, so I think it should probably be renamed / refactored.
i think not. CredentialsInterface does not have any method, its a pure marker interface. and the rest of jackalope needs simple credentials anyways. if we ever come up with other credentials, we would need to think of a new way to have the user id.

but actually, if this class is created by our code, we might ask for the user id here instead, to decouple from the credentials class / interface.