[Feature] add OpenPGP as an encryption option

[achanda]

We currently use wal-g for backups and encrypt files using OpenPGP. We are evaluating barman as a part of CNPG and it would be operationally simpler for us if we can continue to use OpenPGP for encryption.

[Agalin]

This is a huge disadvantage of Barman compared to both WAL-G (which supports symmetric keys through libsodium and OpenPGP) and pgbackrest (which supports symmetric encryption and various SSE modes, including SSE-C requested in #973).