- Uses `tracing` instead of `log` crate
- Uses `tracing-subscriber` instead of `fern`
- This allows us to automatically capture `log` events using
`tracing-subscriber`
- Breaking: Makes `extism::set_log_file` private and only used through
the C API, Rust users should use `tracing-subscriber` to determine which
filters/levels to log.
- Adds `extism::set_log_callback` function to set a callback that can be
used for custom logging from Rust.
- Adds `bool extism_log_custom(const char *level)` and
`extism_log_drain(void (*fn)(const char *s, size_t length)` to the C API
to enable custom sinks in other SDKs
