add tracing feature

[guswynn]

Much of the rust ecosystem uses tracing over log

This branch adds an optional feature tracing to rdkafka that uses tracing log lines instead of log ones. See https://github.com/MaterializeInc/materialize/blob/883710649f95d232ead5872518969a15c1def542/src/kafka-util/src/client.rs#L28-L47 for an example of how people have to work around this, over using the DefaultClientContext

Some notes:

• tracing::enabled! is slightly different than log::log_enabled! in that it doesn't query the tracing subscriber for specifically Event type's. In practice, I have found this to be a vanishingly small difference, but with implement enabled_span and enabled_event tokio-rs/tracing#1900 this pr (after its landed, backported, and released), it could be possible to slightly improve
• tracing log levels are consts, not variants, so I have to do some re-declaration to make them importable to make the enabled! callsites as easy as possible to use
• I am not sure how to test this, other than just cargo test/check, but I need help setting up ci to exercise this

[guswynn]

cc @benesch

[benesch]

Thanks, @guswynn! I wired up CI to at least run cargo check with the new tracing feature enabled.