[Modding] Add GENERIC Structure types

[Arithyce]

Describe the feature you'd like
EDIT FOR CLARITY:
Structure is the value used in research.json for upgrading base structures.
Wall is the value used in research.json for upgrading defense structures.
DEFENSE, WALL, WALL CORNER, GATE, and GENERIC are considered Wall for upgrade purposes, everything else is Structure.

There really needs to be an equivalent of GENERIC for structures (Structure), maybe GENERAL or MISC. Right now, if you want to add a new structure with no functionality, so that you can add functionality through a script, you need to use either DEFENSE or GENERIC, but then your structure will be considered a Wall, which is undesirable.
EDIT FOR CLARITY: For the purposes of applying upgrades through research, this is important.

Describe why do you think it is needed
There is currently no way to add new structures (Structure) with new functionality, while you can add new defenses (Wall) with new functionality. There is an exception in the MISSILE SILO, but that has potentially unwanted traits, such as being packable and explosive.

More information
I can easily add this myself, so i will be creating a PR soon with the addition.
Fixed by: #3777