[ul] Implement generic Extended Negotiation

[pgimeno4d]

Add a Negotiation trait which contains, for the moment, the extended_negotiation method. In future it can also help implement Role Selection negotiation (#731) and possibly other currently unimplemented features like Common Extended Negotiation. Note I'm also interested in #731 and will probably submit a patch if this one is merged, as it will depend on said trait.

This is a case where the client and the server differ substantially. The Negotiation trait is only for the server; the client just adds pairs SOP Class / Byte array. After the association succeeds, they both have the same set of pairs, which they can look up to determine which extended options are active, through the new method extended_negotiation_for(sop_class_uid: &str) or by walking the user_variables().

Everything is quite low-level, to cover for all current and hopefully future versions of the standard. It's easy to send invalid bytes, for instance.

I'm not quite sure if this is a breaking change. It did not break anything in our application, which is a server. The addition of a new generic concerned me a bit.

Edit: Forgot to mention, this obviously depends on #727, otherwise the bytestream is not correct.

[pgimeno4d]

Rebased on top of #727 so that it can be properly tested with that change applied.