Merge RNP / GnuPG handling code into adapters

[ronaldtse]

As with Enmail, we already have two versions of this Engine, one that works with GnuPG and another that works with RNP.

Since they are technically developed sequentially: first GnuPG, then migrated to RNP, we have two separate branches.

The goal of this issue is to combine both branches into the Adapter pattern used in Enmail.

@skalee do you have time for this? Thanks!

[skalee]

I think I do.

[ronaldtse]

Thanks @skalee !