Update Protobuf and add protos for CS2 (alliedmodders#176)

* Replace protobuf 2.6.1 with 3.21.8

* Update/add protobuf libs

* Add CS2 protos

* Remove old csgo/dota protos

* Add versioned protoc bin

* Comment out Valve's `schema` define for now

* Use ENetworkDisconnectionReason

* Fix-up `offsetof` to avoid errors on some Clang versions

Author: psychonic

common/protobuf-2.6.1/python/google/protobuf/__init__.py renamed to .gitmodules

@@ -0,0 +1,19 @@
+import "netmessages.proto";
+
+message C2S_CONNECT_Message {
+	optional uint32 host_version = 1;
+	optional uint32 auth_protocol = 2;
+	optional uint32 challenge_number = 3;
+	optional fixed64 reservation_cookie = 4;
+	optional bool low_violence = 5;
+	optional bytes encrypted_password = 6;
+	repeated .CCLCMsg_SplitPlayerConnect splitplayers = 7;
+	optional bytes auth_steam = 8;
+	optional string challenge_context = 9;
+	optional sint32 use_snp = 10;
+}
+
+message C2S_CONNECTION_Message {
+	optional string addon_name = 1;
+	optional bool use_snp = 2;
+}

common/connectionless_netmessages.proto

@@ -0,0 +1,14 @@
+import "google/protobuf/descriptor.proto";
+
+message CEngineGotvSyncPacket {
+	optional uint64 match_id = 1;
+	optional uint32 instance_id = 2;
+	optional uint32 signupfragment = 3;
+	optional uint32 currentfragment = 4;
+	optional float tickrate = 5;
+	optional uint32 tick = 6;
+	optional float rtdelay = 8;
+	optional float rcvage = 9;
+	optional float keyframe_interval = 10;
+	optional uint32 cdndelay = 11;
+}