Consider using xmltree for implementation and public API

[mxgrey]

This is a follow-up to this discussion which took place in a rmf_site issue ticket.

I'm opening this to facilitate the discussion in a more relevant location, not to mandate that we should follow through with the idea.

@luca-della-vedova brought up some good points about how the SdfPlugin and XmlElement structs allow for a more ergonomic experience for the average SDFormat user.

I think it's worth examining whether we can create some newtypes or some extension traits around the xmltree types to deliver the same ergonomics for users via an xmltree-based API while they can still benefit from the added functionality of xmltree.

I don't consider this to be urgent at all; it's just something to consider if we have the bandwidth and anyone would find it useful.