Log setup functions and `tracing-subscriber` are opt-in with feature `log_setup`

"Log setup for binaries is opt-in with feature log_setup"

re_log depended on tracing-subscriber unconditionally, and every Rerun crate depends on re_log. Because the workspace pins tracing-subscriber at ^0.3.23, a project pinning an earlier 0.3.x could not depend on Rerun at all — even when using it purely as a SDK, with no viewer and no subscriber of its own.

tracing-subscriber now sits behind re_log's existing setup feature. All source-code usage of tracing-subscriber was already gated on this feature.

The rerun crate exposes re_log/setup as a new log_setup feature, off by default. log_setup covers the whole of re_log's application-level logging setup: setup_logging, setup_logging_with_filter, add_log_msg_receiver, LogMsg, Receiver, Sender, FieldValue, PanicOnWarnScope, and LevelFilter. Libraries should probably leave it off and let whoever owns main configure logging.

Binaries that set up logging through the re-exported re_log must now opt in. Without the feature, the call no longer resolves:

error[E0425]: cannot find function `setup_logging` in crate `re_log`
note: found an item that was configured out
      the item is gated behind the `setup` feature