Now that https://www.rfc-editor.org/rfc/rfc9788.html is published we can implement this standard mechanism for header protection instead of what we currently do.

We currently use protected-headers=v1 attribute on the Content-Type which is only handled for Subject in Thunderbird and K-9 Mail. We also extend it to protect more headers, but RFC 9788 is more robust than what we do because it does not have to special case all the headers that must never appear unprotected, but instead ignores all unprotected headers if there are protected headers without merging them.

New mechanism is identified by an hp attribute and is independent of protected-headers.

We first need support for receiving such messages and then can switch to sending them some version later.

We likely don't want to send Legacy Display Element, but need to be able to receive it.