Configurable column_encryption cipher

[baelter]

Are you open to making the cipher used in column_encryption configurable?
aes-256-gcm is not supported by pgcrypto which is a bit inconvenient if you want to decrypt outside of Sequel.

I can give it a go if you think it's a good idea @jeremyevans

[jeremyevans]

I don't plan on making the cipher configurable unless there are cryptographic issues discovered in aes-256-gcm. So if you want to use a different cipher, I recommend you copy the current column_encryption plugin and modify it.