Add missing implementations of then() method

pgpainless-core/src/main/kotlin/org/pgpainless/signature/subpackets/CertificationSubpackets.kt

@@ -6,7 +6,22 @@ package org.pgpainless.signature.subpackets
 
 interface CertificationSubpackets : BaseSignatureSubpackets {
 
-    interface Callback : SignatureSubpacketCallback<CertificationSubpackets>
+    interface Callback : SignatureSubpacketCallback<CertificationSubpackets> {
+        fun then(nextCallback: SignatureSubpacketCallback<CertificationSubpackets>): Callback {
+            val currCallback = this
+            return object : Callback {
+                override fun modifyHashedSubpackets(hashedSubpackets: CertificationSubpackets) {
+                    currCallback.modifyHashedSubpackets(hashedSubpackets)
+                    nextCallback.modifyHashedSubpackets(hashedSubpackets)
+                }
+
+                override fun modifyUnhashedSubpackets(unhashedSubpackets: CertificationSubpackets) {
+                    currCallback.modifyUnhashedSubpackets(unhashedSubpackets)
+                    nextCallback.modifyUnhashedSubpackets(unhashedSubpackets)
+                }
+            }
+        }
+    }
 
     companion object {
 

pgpainless-core/src/main/kotlin/org/pgpainless/signature/subpackets/RevocationSignatureSubpackets.kt

@@ -6,10 +6,32 @@ package org.pgpainless.signature.subpackets
 
 import org.bouncycastle.bcpg.sig.RevocationReason
 import org.pgpainless.key.util.RevocationAttributes
+import org.pgpainless.signature.subpackets.SelfSignatureSubpackets.Callback
 
 interface RevocationSignatureSubpackets : BaseSignatureSubpackets {
 
-    interface Callback : SignatureSubpacketCallback<RevocationSignatureSubpackets>
+    interface Callback : SignatureSubpacketCallback<RevocationSignatureSubpackets> {
+        fun then(
+            nextCallback: SignatureSubpacketCallback<RevocationSignatureSubpackets>
+        ): Callback {
+            val currCallback = this
+            return object : Callback {
+                override fun modifyHashedSubpackets(
+                    hashedSubpackets: RevocationSignatureSubpackets
+                ) {
+                    currCallback.modifyHashedSubpackets(hashedSubpackets)
+                    nextCallback.modifyHashedSubpackets(hashedSubpackets)
+                }
+
+                override fun modifyUnhashedSubpackets(
+                    unhashedSubpackets: RevocationSignatureSubpackets
+                ) {
+                    currCallback.modifyUnhashedSubpackets(unhashedSubpackets)
+                    nextCallback.modifyUnhashedSubpackets(unhashedSubpackets)
+                }
+            }
+        }
+    }
 
     fun setRevocationReason(
         revocationAttributes: RevocationAttributes