Move "User Email header overrides" into a separate module or separate them in the UI

Problem/Motivation

User Email header overrides are not a key functionality of this module and more kind of risky than super helpful.

So I see three possible way to separate them:
1. Move them into a submodule and use HOOK_form_alter to apply them, if enabled
2. Move them into a separate form and tab
3. Add a checkbox "Expert settings" and only show the fieldset / details, if it's enabled.

3 is the most lightweight change, 1 and 2 are not that simple.

Which one should we choose?

Steps to reproduce

Proposed resolution

Discuss
Implement
Write tests for the functionality to work as expected
Release

Remaining tasks

User interface changes

API changes

Data model changes