Support concurrent.future

### Problem Statement

When using Sentry with ThreadPoolExecutor, I couldn't automatically capture errors like in this [issue](https://github.com/getsentry/sentry-python/issues/1234).

I think ThreadPoolExecutor use [this run method](https://github.com/python/cpython/blob/723f4d66982e4d2c54f8e874d6084ab7b2ff5833/Lib/concurrent/futures/thread.py#L53).
```
def run(self):
if not self.future.set_running_or_notify_cancel():
return
try:
result = self.fn(*self.args, **self.kwargs)
except BaseException as exc:
self.future.set_exception(exc)
# Break a reference cycle with the exception 'exc'
self = None
else:
self.future.set_result(result)
```

### Solution Brainstorm

In future.thread, it catches BaseException and handles it by using future.set_exception.
Therefore, I am planning to create FutureIntegration to patch Future.set_exception so that Sentry can handle it.

like this.

```
def setup_once():
from concurrent.futures import Future
from concurrent.futures._base import CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED, InvalidStateError

def sentry_set_exception(self, exception):
with self._condition:
if self._state in {CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED}:
raise InvalidStateError('{}: {!r}'.format(self._state, self))
self._exception = exception
self._state = FINISHED
for waiter in self._waiters:
waiter.add_exception(self)
self._condition.notify_all()
self._invoke_callbacks()

_capture_exception()

Future.set_exception = sentry_set_exception
```

I've tested this process in my local environment and confirmed that it works well. Would it be okay for me to contribute in this way?