Custom headers for all requests

Hello,

Similar to [issue #5821 in the Android repo](https://github.com/nextcloud/android/issues/5821), it would be nice to have possibility to add custom headers via app to all requests.

For example, when nextcloud is hosted behind Pangolin / CloudFlare, we can proxy access via that provider's authentication, which provides additional security that only authorized users will end up on Nextcloud instance.

Thanks for such an amazing OSS project 🙏 ❤️