refactor: separate file watcher from router

[calmini]

continue from #141

Separate the file watcher from router, in the later refactor pr, the interaction will act as below:

class MCPRouter:

    def __init__(self):
        self.client_session_manager: MCPClientSessionManager = session_manager or MCPClientSessionManager.get_instance()
        self.notifier = ConfigUpdateNotifier.get_instance()
        self._unsubscribe = None

    async def initialize_router(self):
        ...
        # subscribe config update
        self._unsubscribe = self.notifier.subscribe(self._handle_config_update)
        ...

    async def _handle_config_update(self, config_type: ConfigType):
        """handles configuration update"""
        logger.info(f"Received configuration update from {config_type}")
        if config_type == ConfigType.FILE:
            self.profile_manager.reload()
            servers_wait_for_update = self.get_unique_servers()
            servers_add, servers_remove = await self.client_session_manager.update_sessions(servers_wait_for_update)
            logger.info(f"Updated servers: {','.join(servers_add)} added, {','.join(servers_remove)} removed")
        elif config_type == ConfigType.CLOUD:
            logger.warning("Cloud config update is not supported yet")

and inside of app scope

@asynccontextmanager
async def lifespan(app):
    ...
    global router
    router = MCPRouter(session_manager=client_session_manager)
    if RELOAD:
        global watcher
        watcher = FileConfigWatcher(router.profile_manager.profile_path)
        await watcher.start()
    ...

[JoJoJoJoJoJoJo]

looks a little strange to return the unsubscribe function

[calmini]

just for convenience to call when router shutdown

unsubscribe = notifier.subscribe(callback)
# when shutdown
unsubscribe()