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_test.go

@@ -0,0 +1,56 @@
+package helper_test
+
+import (
+	"testing"
+
+	"github.com/ProtonMail/gopenpgp/v2/crypto"
+	"github.com/ProtonMail/gopenpgp/v2/helper"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestEncryptPGPMessageToAdditionalKey(t *testing.T) {
+	keyA, err := crypto.GenerateKey("A", "a@a.a", "x25519", 0)
+	if err != nil {
+		t.Fatal("Expected no error when generating key, got:", err)
+	}
+
+	keyB, err := crypto.GenerateKey("B", "b@b.b", "x25519", 0)
+	if err != nil {
+		t.Fatal("Expected no error when generating key, got:", err)
+	}
+
+	keyRingA, err := crypto.NewKeyRing(keyA)
+	if err != nil {
+		t.Fatal("Expected no error when creating keyring, got:", err)
+	}
+	keyRingB, err := crypto.NewKeyRing(keyB)
+	if err != nil {
+		t.Fatal("Expected no error when creating keyring, got:", err)
+	}
+
+	message := crypto.NewPlainMessageFromString("plain text")
+	// Encrypt towards A
+	ciphertext, err := keyRingA.Encrypt(message, nil)
+	if err != nil {
+		t.Fatal("Expected no error when encrypting, got:", err)
+	}
+	ciphertextSplit, err := ciphertext.SplitMessage()
+	if err != nil {
+		t.Fatal("Expected no error when splitting message, got:", err)
+	}
+	// Also encrypt the message towards B
+	if err := helper.EncryptPGPMessageToAdditionalKey(ciphertextSplit, keyRingA, keyRingB); err != nil {
+		t.Fatal("Expected no error when modifying the message, got:", err)
+	}
+
+	// Test decrypt with B
+	decrypted, err := keyRingB.Decrypt(
+		ciphertextSplit.GetPGPMessage(),
+		nil,
+		0,
+	)
+	if err != nil {
+		t.Fatal("Expected no error when decrypting, got:", err)
+	}
+	assert.Exactly(t, message.GetString(), decrypted.GetString())
+}