Websocket refactoring: allow lazy async connections (#78)

In preparation for migrating the async API over to using
the AsyncWebsocketHandler API (instead of using the httpx-ws
instance API directly), rework the task scheduling so the
websocket handlers run inside one task group per client.

Since the task manager's additional tasks group will own the
handlers, it will no longer matter exactly which client coroutine
triggers setting up the connection (once the async API migrates
to the new imlementation interface).

CI lock file updates are due to refactoring eliminating the
dependency on the "exceptiongroups" project.