Support encrypt to public keys

[clintfred]

The current DocumentOps::document_encrypt allows the caller to specify user/groups by ID. The service then looks up the public keys for the users/groups for encryption.

It would be nice if we provided an interface that also allowed encryption directly to public keys (if the caller already has them). This will save 2 calls to the service.

An internal API already exists with (almost) this functionality. See internal::document_api::recrypt_document