[Unity-plugin] Context menu for creating Unity terrain components

[Balint-H]

A QoL addition to create and align Unity terrain with hfield geoms. Also includes tweaks to the way the MJCF is constructed (previously the local Y position of the terrain was used as the basehieght, now it can be separately defined in the HField property of the geom).

[Balint-H]

@erez-tom Once you are available again could you please merge this in? Would fix #1930.