feat(Core/Chat): Provide a fully-formed protocol for addons to intera… (#19305)

* feat(Core/Chat): Provide a fully-formed protocol for addons to interact with GM commands

* Send success/fail state, allow interleaving, and indicate end of output. Add framework for supporting non-human-readable output in commands.

* cherry-pick commit (508c9d2fc1)

This PR implements a well-formed protocol for addons to communicate with the server, outlined below:

- All communication happens over the addon channel (`LANG_ADDON` in terms of the core, `CHAT_MSG_ADDON`/`SendAddonMessage` for the client). The prefix used for all messages is `AzerothCore` (in client terms - in core terms, every message starts with `AzerothCore\t`).
- In each message, the first character is the opcode. The following four characters are a unique identifier for the invocation in question, and will be echoed back by the server in every message related to that invocation. Following is the message body, if any.
- The following opcodes are supported:
    - Client to server:
        - `p` - Ping request. The core will always respond by ACKing with the passed identifier. No body.
        - `i` or `h` - Command invocation. The message body is the command text without prefix. `i` requests machine-readable output, `h` requests human-readable.
    - Server to client:
        - `a` - ACK. The first message sent in response to any invocation (before any output). No body.
        - `m` - Message. Sent once per line of output the server generates. Body = output line.
        - `o` - OK. Indicates that the command finished processing with no errors. No body.
        - `f` - Failed. Indicates that command processing is done, but there was an error. No body.

Expected overhead is minimal, and this integrates seamlessly with existing command scripts (no changes necessary).

PS: There's also a client-side addon library that exposes this protocol in a developer-friendly way over at https://github.com/azerothcore/LibAzerothCore-1.0

---------

Co-authored-by: Treeston <14020072+Treeston@users.noreply.github.com>

src/server/game/Chat/Chat.h

@@ -168,4 +168,24 @@ private:
     Print* m_print;
 };
 
+class AC_GAME_API AddonChannelCommandHandler : public ChatHandler
+{
+    public:
+        using ChatHandler::ChatHandler;
+        bool ParseCommands(std::string_view str) override;
+        void SendSysMessage(std::string_view str, bool escapeCharacters) override;
+        using ChatHandler::SendSysMessage;
+        bool IsHumanReadable() const override { return humanReadable; }
+
+    private:
+        void Send(std::string const& msg);
+        void SendAck();
+        void SendOK();
+        void SendFailed();
+
+        std::string echo;
+        bool hadAck = false;
+        bool humanReadable = false;
+};
+
 #endif