Consider adding `ClientManager` from `sparqlx`

[lu-pl]

The httpx client manager from sparqlx could be generally useful and would fit lupl as a collection of utils.

The class could be named lupl.HttpxClientManager.
One could also further investigate implementing the context manager protocol directly; currently the class exposes context and acontext manager - implementing __enter__/__exit__ directly on the class brings interesting problems, e.g. contextvars might be needed.