fix(tracing) : Add trace to __all__ in top-level __init__.py (#2401)

Currently, using the decorator form of `trace` like this (as mentioned in the docs[1]):

```
import sentry_sdk

@sentry_sdk.trace
def do_stuff():
```

causes mypy to throw a `Module "sentry_sdk" does not explicitly export attribute "trace"  [attr-defined]` error. This adds `trace` to the top-level `__init__.py`'s `__all__` so mypy sees it as being officially exported and stops throwing the error.

[1] https://docs.sentry.io/platforms/python/performance/instrumentation/custom-instrumentation/#using-a-decorator-1

Author: lobsterkatie

sentry_sdk/__init__.py

@@ -39,6 +39,7 @@
     "get_traceparent",
     "get_baggage",
     "continue_trace",
+    "trace",
 ]
 
 # Initialize the debug support after everything is loaded