fix: relax Client constructor cfg typing

[anzzyspeaksgit]

Closes #134

Description

This addresses the issue with the overly strict type annotation on the Client constructor's cfg parameter. Since dict is invariant, mypy correctly rejects valid nested configurations passed as dictionaries (e.g. dict[str, dict[str, object]]).

By changing the type hint from Optional[dict[str, object]] to Optional[Mapping[str, Any]], it now safely accepts user-supplied dictionaries containing nested configuration dictionaries without raising mypy type errors.

Using Mapping instead of dict ensures covariance, which fixes the problem.