Add the ability to set the "In-Reply-To" header

[castarco]

As of today, it is very difficult (perhaps impossible) to use the Tuta desktop client to participate in Linux email lists "posthoc" (once the discussion already has many messages).

Being able to set the "In-Reply-To" header would make it possible to join the list after a discussion already started and to participate in it.

For reference, one can check the suggestions provided by one of the web mirrors ( https://lore.kernel.org/lkml/7dwjsfnrxyxewrxsyznkl6kbgilnfisom7igpeyesmihktejqt@njz4xjtpcgw5/#t ) ... None of them are easy to apply when relying on Tuta mailing services (I suspect some of them aren't just difficult, but impossible, but I won't subject myself to hours of torture trying it).