Add ability to restore RandomNumberGenerator state

[Xrayez]

3.2 version of #44089.

get_seed() still returns the previous state and not the initial seed, because users may rely on this behavior for resetting the state in 3.2. Documented this is going to be fixed in 4.0.

func _ready():
	var rng = RandomNumberGenerator.new()
	var initial_state = rng.state
	print("Initial ", rng.state)
	print(rng.randf())
	print(rng.randf())
	print(rng.randf())
	print("Current ", rng.state)
	print("Reset to initial state...")
	rng.state = initial_state
	print(rng.randf())
	print(rng.randf())
	print(rng.randf())
	print("Current ", rng.state)

Initial -6332740101472756719
0.621225
0.978975
0.367289
Current -6028506256414837201
Reset to initial state...
0.621225
0.978975
0.367289
Current -6028506256414837201

[akien-mga]

Thanks!