Reload the pyo3-log config when the Python logging config changes. (#14976)

Since pyo3-log is initialized very early in the Python start-up
it caches the state of the loggers before they're fully initialized
(and thus are essentially disabled). Whenever we reload the
logging configuration we now also tell pyo3-log to discard
any cached logging configuration it has; it will refetch the
current logging configuration from Python at the next point
it logs.

This fixes Rust log lines not appearing in the homeserver logs.

stubs/synapse/synapse_rust/__init__.pyi

@@ -1,2 +1,3 @@
 def sum_as_string(a: int, b: int) -> str: ...
 def get_rust_file_digest() -> str: ...
+def reset_logging_config() -> None: ...