Add Alpine package registry (#23714)

This PR adds an Alpine package registry. You can follow [this
tutorial](https://wiki.alpinelinux.org/wiki/Creating_an_Alpine_package)
to build a *.apk package for testing.

This functionality is similar to the Debian registry (#22854) and
therefore shares some methods. I marked this PR as blocked because it
should be merged after #22854.


![grafik](https://user-images.githubusercontent.com/1666336/227779595-b76163aa-eea1-4a79-9583-775c24ad74e8.png)

---------

Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

modules/util/keypair.go

@@ -4,10 +4,13 @@
 package util
 
 import (
+	"crypto"
 	"crypto/rand"
 	"crypto/rsa"
 	"crypto/x509"
 	"encoding/pem"
+
+	"github.com/minio/sha256-simd"
 )
 
 // GenerateKeyPair generates a public and private keypair
@@ -43,3 +46,16 @@ func pemBlockForPub(pub *rsa.PublicKey) (string, error) {
 	})
 	return string(pubBytes), nil
 }
+
+// CreatePublicKeyFingerprint creates a fingerprint of the given key.
+// The fingerprint is the sha256 sum of the PKIX structure of the key.
+func CreatePublicKeyFingerprint(key crypto.PublicKey) ([]byte, error) {
+	bytes, err := x509.MarshalPKIXPublicKey(key)
+	if err != nil {
+		return nil, err
+	}
+
+	checksum := sha256.Sum256(bytes)
+
+	return checksum[:], nil
+}