Define the `E2EE-To-All` header

Daniel Huigensrequested to merge
twisstle/cleartext-copy:define-e2ee-to-all into main

This MR proposes the E2EE-To-All header as a concrete solution to the problems outlined in this draft, and provides guidance to MUAs for generating and handling the E2EE-To-All header.

It also removes the other options previously considered in this draft.

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