Add "related_underground_pipe" to pipe prototype definitions

[rimbas]

For feature parity with Prototype/TransportBelt#related_underground_belt and LuaEntityPrototype#related_underground_belt. It doesn't exist because pipes don't have smart drag quick replace function but I believe there is still a benefit to having it for inter mod compatibility.

I already use LuaEntityPrototype#related_underground_belt to avoid hardcoding name lists or doing string pattern matching, so also having it for pipes would be nice.

[DaleStan]

Does that still do what you want when the fluid system is extended by mods like Advanced Fluid Handling?

[rimbas]

Does that still do what you want when the fluid system is extended by mods like Advanced Fluid Handling?

Ideally pipes with custom behaviour wouldn't define/be defined as a related pipe but we can read fluidbox connections so its possible to filter them if they don't behave as expected.