Add async client objects

[danielballan]

• [x] Remove tiled/client/_async_bridge.py, a temporary hack that runs the httpx.AsyncClient on a thread to make ASGI work.
• [x] Rework the refresh flow code to httpx's sans I/O mechanism for pluggable authentication.
• [ ] Rework the rest of the client code following a pattern similar to the authentication one, aiming for sans I/O utilities, breaking out sync/async cases as needed to handle I/O. I'm not sure whether following httpx in using subclasses is best here. Consider composition.
• [x] Use this opportunity to make tiled.client.node.Node inherit from tiled.client.base.BaseClient.