Prepare release 2.7.5 with packet API (#269)

Adds the following API functions:
- API to get signature key IDs for mobile:
	func (msg *PGPMessage) GetHexSignatureKeyIDsJson() []byte

- API to get encryption key IDs for mobile:
	func (msg *PGPMessage) GetHexEncryptionKeyIDsJson() []byte

- API to get the number of key packets in a PGP message:
	func (msg *PGPSplitMessage) GetNumberOfKeyPackets() (int, error)

- API in package `helper` to encrypt a PGP message to an additional key:
	func EncryptPGPMessageToAdditionalKey(messageToModify *crypto.PGPSplitMessage, keyRing *crypto.KeyRing, additionalKey *crypto.KeyRing) error

helper/message.go

@@ -0,0 +1,22 @@
+package helper
+
+import "github.com/ProtonMail/gopenpgp/v2/crypto"
+
+// EncryptPGPMessageToAdditionalKey decrypts the session key of the input PGPSplitMessage with a private key in keyRing
+// and encrypts it towards the additionalKeys by adding the additional key packets to the input PGPSplitMessage.
+// If successful, new key packets are added to message.
+// * messageToModify : The encrypted pgp message that should be modified
+// * keyRing         : The private keys to decrypt the session key in the messageToModify.
+// * additionalKey   : The public keys the message should be additionally encrypted to.
+func EncryptPGPMessageToAdditionalKey(messageToModify *crypto.PGPSplitMessage, keyRing *crypto.KeyRing, additionalKey *crypto.KeyRing) error {
+	sessionKey, err := keyRing.DecryptSessionKey(messageToModify.KeyPacket)
+	if err != nil {
+		return err
+	}
+	additionalKeyPacket, err := additionalKey.EncryptSessionKey(sessionKey)
+	if err != nil {
+		return err
+	}
+	messageToModify.KeyPacket = append(messageToModify.KeyPacket, additionalKeyPacket...)
+	return nil
+}