[Net] Basic extensible MultiplayerAPI spawn/despawn.

`PackedScene`s can be configured to be spawnable via a new
`MultiplayerAPI.spawnable_config` method.
They can be configured either to be spawned automatically when coming
from the server or to always require verification.
Another method, `MultiplayerAPI.send_spawn` lets you request a spawn on
the remote peers.
When a peer receive a spawn request:
- If it comes from the server and the scene is configured as
  `SPAWN_MODE_SERVER`:
  - Spawn the scene (instantiate it, add it to tree).
  - Emit signal `network_spawn`.
- Else:
  - Emit signal `network_spawn_request`.

In a similar way, `despawn`s are handled automatically in
`SPAWN_MODE_SERVER`.

In `SPAWN_MODE_SERVER`, when a new client connects it will also receive,
from the server all the spawned (and not yet despawned) instances.

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