Support OAuth for Email Transport

Add OAuth support for email transport authentication settings.

## What?
Currently, Graylog only supports basic SMTP authentication which is being deprecated by the most common mail exchange service our customers use- M365 Exchange. According to their bulletin, support for basic SMTP auth will be going away March 1st 2026 [ref](https://techcommunity.microsoft.com/blog/exchange/exchange-online-to-retire-basic-auth-for-client-submission-smtp-auth/4114750).

There is an alternative for customers sending emails **internal** to their tenant however, called "High Volume Email for M365." This would probably cover many cases, but OAuth is clearly the authentication trend in the industry and we already support it in other aspects of Graylog already.

## Why?
Will cause email outage for customers who do not (or cannot) switch to the High Volume Email alternative, or whose Graylog environment sends emails to recipients outside their M365 tenant.

## Your Environment

* Graylog Version: 6.3