Public API for manual replay control

Similarly to [web replays](https://docs.sentry.io/platforms/javascript/session-replay/understanding-sessions/#manually-starting-replay), we should provide public API for controlling replay behavior manually:
- `start/startBuffering` should start recording a replay regardless of sample rates set, with the corresponding mode
- `stop` should stop the replay and send it to sentry
- `flush` should flush all pending segments/frames to sentry

### `start/stop` current Replay
- [x] Apple impl: https://github.com/getsentry/sentry-cocoa/pull/4414
- [ ] Apple still has to provide startBuffering https://github.com/getsentry/sentry-cocoa/issues/6126
- [ ] Apple docs: (TODO)
- [ ] Android (TODO)
- [x] https://github.com/getsentry/sentry-react-native/issues/4441
- [x] https://github.com/getsentry/sentry-dart/issues/2558

There's also interest in permanently deactivating SR after SDK init, e.g. as described here: - https://github.com/getsentry/sentry-cocoa/issues/4833