Allow custom "reply to" addresses in emails

It's now possible to set post authors to appear as "reply to" emails:

An improvement would be adding an option to allow any email to be a "reply to" address.

P2 convo peKye1-NC-p2
Prototype https://github.com/Automattic/jetpack/pull/37041

Related:
- https://github.com/Automattic/jetpack/issues/37354
- https://github.com/Automattic/jetpack-roadmap/issues/1410
- https://github.com/Automattic/jetpack-roadmap/issues/1409

Start by reviewing the existing post-author “reply to” behavior shown in the issue and the prototype in pull request 37041. Compare the requested arbitrary email option with related issues 37354 and roadmap items 1410 and 1409; done means users can configure any intended reply-to address safely and the behavior is covered by the project’s existing checks.

Written by the indexing model from the issue text.