added firebase and rudimentary leaderboard support

addons/godot-firebase/auth/providers/facebook.gd

@@ -0,0 +1,21 @@
+class_name FacebookProvider 
+extends AuthProvider
+
+func _init(client_id: String,client_secret: String):
+	randomize()
+	set_client_id(client_id)
+	set_client_secret(client_secret)
+	
+	self.redirect_uri = "https://www.facebook.com/v13.0/dialog/oauth?"
+	self.access_token_uri = "https://graph.facebook.com/v13.0/oauth/access_token"
+	self.provider_id = "facebook.com"
+	self.params.scope = "public_profile"
+	self.params.state = str(randf_range(0, 1))
+	if Utilities.is_web():
+		self.should_exchange = false
+		self.params.response_type = "token"
+	else:
+		self.should_exchange = true
+		self.params.response_type = "code"
+		
+