[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.
can you create a new issue about this? we might want a pluggable etag builder as this can be something that requires custom logic. then we provide a straightforward default impl that is used if no other is specified.