Make rich dependency optional.

[racinmat]

Is your feature request related to a problem? Please describe.
If I have rich installed, structlog provides rectangular stack traces, and it can' be easily disabled: https://www.structlog.org/en/stable/console-output.html

Describe the solution you'd like
Have dependency optional, as it's e.g. in https://github.com/modelcontextprotocol/python-sdk/blob/v1.13.1/pyproject.toml#L39

Describe alternatives you've considered
Abandoning this library, because it's making me crazy.

Additional context
No other needed.