Add configurable X/Z offset

[InstantlyMoist]

Overview

Simply adds a configurable X/Z offset, as I needed that and might be useful to others.

Description

It renames 'offset' to 'offset_y' and adds 'offset_x' + 'offset_z' as I didnt want the hologram to appear on top of the road, but rather the border.

Submitter Checklist

• Make sure you are opening from a topic branch (/feature/fix/docs/ branch (right side)) and not your main branch.
• Ensure that the pull request title represents the desired changelog entry.
• New public fields and methods are annotated with @since TODO.
• I read and followed the contribution guidelines.

[PierreSchwang]

The solution is generally fine, though I'm not the biggest fan of simply dropping a configuration property.

I am quite unsure if it would be better to make the sign location configurable in PlotSquared itself (which would propagate to HoloPlots) - or if those should be separate.