[Python][FlightRPC] Some flight classes not exported as public

### Describe the bug, including details regarding any error messages, version, and platform.

For example, `_flight.ServerAuthSender` is not exposed in `pyarrow.flight`

So we cannot write

```
import pyarrow.flight as flight

def authenticate(self, outgoing: flight.ServerAuthSender, incoming: flight.ServerAuthReader):
pass

```

without the IDE/linter erroring out that ServerAuthSender is not a known member of module `flight`

There's also no documentation for it in https://arrow.apache.org/docs/python/generated/pyarrow.flight.ServerAuthHandler.html

### Component(s)

FlightRPC, Python