[Core] Add a cli flag for setting a max size of message cache (#3474)

* Add an arg in cli to change message cache size

* Add an arg in cli to change message cache size

* Changelog

* Actually pass None in message_cache_size

* Update cli.py

* Add a cli arg to disable message cache.

* Add a cli arg to disable message cache.

* well go away you useless

* you actually are an int

* Check if message cache is higher than 0 when set it.

* Use sys.maxsize as max cache size.

* Update cli.py

* Add bot.max_messages property.

* typos

* 🤦

* style

redbot/core/bot.py

@@ -149,6 +149,12 @@ class RedBase(
         if "command_not_found" not in kwargs:
             kwargs["command_not_found"] = "Command {} not found.\n{}"
 
+        message_cache_size = cli_flags.message_cache_size
+        if cli_flags.no_message_cache:
+            message_cache_size = None
+        kwargs["max_messages"] = message_cache_size
+        self._max_messages = message_cache_size
+
         self._uptime = None
         self._checked_time_accuracy = None
         self._color = discord.Embed.Empty  # This is needed or color ends up 0x000000
@@ -271,6 +277,10 @@ class RedBase(
     def colour(self) -> NoReturn:
         raise AttributeError("Please fetch the embed colour with `get_embed_colour`")
 
+    @property
+    def max_messages(self) -> Optional[int]:
+        return self._max_messages
+
     async def allowed_by_whitelist_blacklist(
         self,
         who: Optional[Union[discord.Member, discord.User]] = None,