Input O365 not consistently using configured Proxy

User follows a policy to connect to the Internet over a Proxy only (no direct connections). Proxy configuration properly done in server.conf; O365 Input cannot be configured anyway, users can see (blocked) connection attempts on their firewall from the Graylog system.

## Expected Behavior

Once the proxy configuration for Graylog has been correctly implemented, the O365 Input should work without issues using the Proxy only.

## Current Behavior

When setting up the O365 Input, it starts an intial authentication attempt towards login.microsoft.com (20.190.159.73, 40.126.31.67, 40.126.32.76,...) without using the configured proxy server. Setup fails as the connection is blocked.

IF the connection is not blocked, the Input THEN connects to manage.office.com using the proxy server as expected.

## Possible Solution

## Steps to Reproduce (for bugs)

1. configure O365 Input in a proxy-only environment

## Context

## Your Environment

* Graylog Version: 5.0.3
* OpenSearch Version: 2.5.0
* MongoDB Version: 6.0