Make log-category for ext logging feature configurable

It would be nice to have the ext logging feature use a given logcategory, rather than using the service name.

That way it's easier to log jax-rs and jax-ws payloads to distinct files w/o having to configure too much, and especially when jax-rs/-ws services cannot be separated by packagename.

Lastly the servicename etc is available through the MDC values