PGPMessage.decrypt and PGPKey.decrypt should offer the ability to return a session key to the caller

[dkg]

This is the converse operation of #389 -- when decrypting, it can be useful to extract a session key from the decryption, to store it for later reuse.

This avoids asymmetric operations (when using PKESK), and S2K operations (when using SKESK) in the future, and also permits tricks like destruction of decryption-capable keys without losing the message itself, or other MUA optimizations (see for example index.decrypt from notmuch-config).

To make this possible, it would be nice if the two high-level decryption operations (PGPMessage.decrypt for passphrase-based decryption, and PGPKey.decrypt for secret-key-based decryption) could somehow return a session key to the user.