AdditionalHeaders not applied on authorize request

## Issue

I am tyring to set up an authorize call, using serviceConfiguration to define authorization and token endpoints. I additionally want to send a header, but I've noticed that additionalHeaders only applies when on the first call to `./well-known/openid-configuration`, and not to the subsequent call to `/authorize`

setup:
```
const config = {
issuer:
authorizeURL,
additionalHeaders: { key: value },
customHeaders: { authorize: { key: value } },
...otherProps
};
```

Is there any workaround for this to send a header in the authorization call?

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## Environment

* **Platform that you're experiencing the issue on**: `both`
* **Your `react-native` Version**: `e.g. 0.74.5`
* **Your `react-native-app-auth` Version**: `e.g. 8.0.0`
* **Are you using Expo?** No