Add bot.get_or_fetch_user/member methods (#4403)

* add 2 get_or_fetch methods

* style

* local tox didnt scream and blow up

* ewh extra space is ugly

* Jack meant this

* I think jack wanted this.

* Nope Jack desired this

* aaaaaaaaaaaaa Jack just say you are reviewing it come on

* add get_or_fetch_member

* Update redbot/core/bot.py

Co-authored-by: jack1142 <6032823+jack1142@users.noreply.github.com>

* Wubba Lubba Dub Dub

* woa!! this one was really HARD! (sweats)

* fiiiiire

Co-authored-by: jack1142 <6032823+jack1142@users.noreply.github.com>

redbot/core/bot.py

@@ -667,6 +667,67 @@ class RedBase(
 
         return self._color
 
+    async def get_or_fetch_user(self, user_id: int) -> discord.User:
+        """
+        Retrieves a `discord.User` based on their ID.
+        You do not have to share any guilds
+        with the user to get this information, however many operations
+        do require that you do.
+
+        .. warning::
+
+            This method may make an API call if the user is not found in the bot cache. For general usage, consider ``bot.get_user`` instead.
+
+        Parameters
+        -----------
+        user_id: int
+            The ID of the user that should be retrieved.
+
+        Raises
+        -------
+        Errors
+            Please refer to `discord.Client.fetch_user`.
+
+        Returns
+        --------
+        discord.User
+            The user you requested.
+        """
+
+        if (user := self.get_user(user_id)) is not None:
+            return user
+        return await self.fetch_user(user_id)
+
+    async def get_or_fetch_member(self, guild: discord.Guild, member_id: int) -> discord.Member:
+        """
+        Retrieves a `discord.Member` from a guild and a member ID.
+
+        .. warning::
+
+            This method may make an API call if the user is not found in the bot cache. For general usage, consider ``discord.Guild.get_member`` instead.
+
+        Parameters
+        -----------
+        guild: discord.Guild
+            The guild which the member should be retrieved from.
+        member_id: int
+            The ID of the member that should be retrieved.
+
+        Raises
+        -------
+        Errors
+            Please refer to `discord.Guild.fetch_member`.
+
+        Returns
+        --------
+        discord.Member
+            The user you requested.
+        """
+
+        if (member := guild.get_member(member_id)) is not None:
+            return member
+        return await guild.fetch_member(member_id)
+
     get_embed_colour = get_embed_color
 
     # start config migrations