[stable30] feat: mail provider settings

[backportbot[bot]]

Backport of #48134

Warning, This backport's changes differ from the original and might be incomplete ⚠️

Todo

• [ ] Review and resolve any conflicts

• [ ] Amend HEAD commit to remove the line stating to skip CI

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[kesselb]

Hi,

This is my usual approach for handling backports with conflicts:

gh pr checkout 49254 -f git reset --hard HEAD~2 git clean -i git cherry-pick 3e870695bc4989e5e2c6f59c44ed5dbd501a7673

and then resolve the conflicts one by one.

There was a lot of automated refactoring/reformatting meanwhile :see_no_evil:

[SebastianKrupinski]

@kesselb Thank you, I was going to work on this today.