Add multiplayer-bomber example.

[Yarwin]

closes: #903
A demo adapted from: https://github.com/godotengine/godot-demo-projects/tree/master/networking/multiplayer_bomber

I took few liberties, mostly related to caching references to nodes in the scene tree by using OnReady, using signals instead of parsing tree in process, and declaring gamestate autoload as engine singleton.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-910

[ValorZard]

Still getting this error on this pull request:

  <C++ Error>    Condition "true" is true. Continuing.
  <C++ Source>   modules/multiplayer/scene_replication_interface.cpp:782 @ on_delta_receive()

Additionally, for some reason the tile map isn't showing up properly

E 0:00:21:0723   create_tile: Cannot create tile. The tile is outside the texture or tiles are already present in the space the tile would cover.
  <C++ Error>    Condition "!room_for_tile" is true.
  <C++ Source>   scene/resources/2d/tile_set.cpp:4963 @ create_tile()

On further testing, this additional error popped up, which is probably a typo

E 0:00:10:0292   get_state: Property '.:player_idx' not found.
  <C++ Error>    Condition "!valid" is true. Returning: ERR_INVALID_DATA
  <C++ Source>   modules/multiplayer/multiplayer_synchronizer.cpp:167 @ get_state()

[Yarwin]

ah, my bad – I forget to update property in the editor in the gdext project (done it only on a copy outside the repo 😬)

output.mp4

[ValorZard]

Update: everything works now, but the missing tilemap issue is still there