BE: Respect proxy settings for OAuth requests

[Haarolean]

provectus/kafka-ui#4114

Impl details and recommendations: provectus/kafka-ui#4114 (comment)

UPD: spring-projects/spring-security#13274 is resolved and should be released as a part of spring-security 6.3.0

[neoXfire]

Hi there !

We are currently facing the same issue in my company so we have interest for a fix. I can contribute if needed 😃.

Bad news, even if the issue you linked was resolved, i think there were multiple changes on that enhancement proposal and recently the new methods to customize the WebClient were reverted 😞

See this issue : spring-projects/spring-security#14178

[Haarolean]

@neoXfire contributions are welcome, see our contributing guide as a start