Add a devtools option

When testing features it would be useful to have the ability to see and flip flags for that instance interactively.

```
import FlagsmithToolbar from 'flagsmith/toolbar'
FlagsmithToolbar.init(flagsmithInstance)
FlagsmithToolbar.show()
FlagsmithToolbar.hide()
```

Would inject an overlay into the site where you could override any feature flag.

## Notes
- We can use the generated types to allow users to select values to change the feaure to
- Users will authenticate via GitHub