consider an "add key" feature

[teemukataja]

The reencrypt feature https://github.com/neicnordic/crypt4gh/tree/master#re-encrypt-files will completely replace the header and keys. Which means that if user wants to just add one extra key to an existing header, they have to have all the old public keys at hand as well.

Investigate if it would be feasible to have a feature for adding a new key to an existing header, without overwriting the header completely.

https://github.com/neicnordic/crypt4gh/blob/master/streaming/out.go#L79-L98