added firebase and rudimentary leaderboard support

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

@@ -0,0 +1,14 @@
+class_name GitHubProvider 
+extends AuthProvider
+
+func _init(client_id: String,client_secret: String):
+    randomize()
+    set_client_id(client_id)
+    set_client_secret(client_secret)
+    self.should_exchange = true
+    self.redirect_uri = "https://github.com/login/oauth/authorize?"
+    self.access_token_uri = "https://github.com/login/oauth/access_token"
+    self.provider_id = "github.com"
+    self.params.scope = "user:read"
+    self.params.state = str(randf_range(0, 1))
+    self.params.response_type = "code"