make preemptive authentication work for proxy connections

## Expected Behavior
All supported proxy methods by OkHttp should work in Graylog.

## Current Behavior

```
2020-12-01T09:34:10.020+01:00 WARN [OkHttpClientProvider$ProxyAuthenticator] Graylog only supports the "basic" authentication scheme but the proxy server asks for one of the following: [okhttp-preemptive]
```

## Possible Solution
Updating OkHttp and enable the feature, since version 3.12 this preemptive auth is supported ( https://github.com/square/okhttp/issues/2435#issuecomment-438519500 )

## Your Environment
* Graylog Version: 3.3 / 4.0

[z#844364]