Add OSM XML support

[Stolexiy]

Add support for OSM XML format and related Diff functionality

Given the distinct structures of regular XML output and the Diff XML format, the' Result' struct has been extended to support Diff data.

Now, go-overpass expects XML format by default to comply with the Overpass API

Your review and feedback are greatly appreciated.

[ahuret]

Hello @serjvanilla :wave: Would you have time to review this new feature ? :D