Add manual controls for Session Tracking

**OS:**

- [ ] Windows
- [X] MacOS
- [ ] Linux

**Platform:**

- [X] iOS
- [X] Android

**SDK:**

- [X] `@sentry/react-native` (>= 1.0.0)
- [ ] `react-native-sentry` (<= 0.43.2)

**SDK version:** 1.4.5

`react-native` version: 0.62.2

**Are you using Expo?**

- [ ] Yes
- [X] No

**Are you using **[**sentry.io**]()** or on-premise?**

- [X] [sentry.io]() (SaaS)
- [ ] on-premise

**If you are using **[**sentry.io**]()**, please post a link to your issue so we can take a look:**

\[Link to issue\]

**Configuration:**

(`@sentry/react-native`)

```
Sentry.init({
dsn: 'https://...@sentry.io/...'
// other options
release: `${appName}@${appVersion} (${buildNumber})`,
enableAutoSessionTracking: true,
sessionTrackingIntervalMillis: 30000,
});
```

---

**I have following issue:**

\[Description\]
Hi, I have an app that purposefully runs in the background when a user starts a "session" with us. I would like to use the session tracking features, but I need to prevent the session from ending while the user's session with us is active.

I'm not aware of any manual way of tracking the session, nor that there's any option to change this configuration at runtime.