[Core] Dispatch on_red_api_tokens_update event on api keys update (#3146)

* feat: dispatch `on_red_api_tokens_update` event on api keys update

* docs: add event reference in Shared API Keys docs

* chore(changelog): add tonwcrier entries

* fix: wrap dispatched api tokens in MappingProxyType

* docs: reflect change of type change to read-only Mapping

redbot/core/bot.py

@@ -8,6 +8,7 @@ from enum import Enum
 from importlib.machinery import ModuleSpec
 from pathlib import Path
 from typing import Optional, Union, List, Dict, NoReturn
+from types import MappingProxyType
 
 import discord
 from discord.ext.commands import when_mentioned_or
@@ -582,6 +583,7 @@ class RedBase(commands.GroupMixin, commands.bot.BotBase, RPCMixin):  # pylint: d
 
         async with self._config.custom(SHARED_API_TOKENS, service_name).all() as group:
             group.update(tokens)
+        self.dispatch("red_api_tokens_update", service_name, MappingProxyType(group))
 
     async def remove_shared_api_tokens(self, service_name: str, *token_names: str):
         """