add new codec for membership changes

Author: nplasterer

library/src/main/java/org/xmtp/android/library/codecs/GroupMembershipChangeCodec.kt

@@ -0,0 +1,49 @@
+package uniffi.xmtpv3.org.xmtp.android.library.codecs
+
+import com.google.gson.GsonBuilder
+import com.google.protobuf.kotlin.toByteStringUtf8
+import org.xmtp.android.library.codecs.ContentCodec
+import org.xmtp.android.library.codecs.ContentTypeId
+import org.xmtp.android.library.codecs.ContentTypeIdBuilder
+import org.xmtp.android.library.codecs.EncodedContent
+
+val ContentTypeGroupMembershipChange = ContentTypeIdBuilder.builderFromAuthorityId(
+    "xmtp.org",
+    "group_membership_change",
+    versionMajor = 1,
+    versionMinor = 0
+)
+
+data class GroupMembershipChanges(
+    val membersAdded: List<MembershipChange>,
+    val membersRemoved: List<MembershipChange>,
+    val installationsAdded: List<MembershipChange>,
+    val installationsRemoved: List<MembershipChange>,
+)
+
+data class MembershipChange(
+    val installationIds: List<ByteArray>,
+    val accountAddress: String,
+    val initiatedByAccountAddress: String,
+)
+
+data class GroupMembershipChangeCodec(override var contentType: ContentTypeId = ContentTypeGroupMembershipChange) :
+    ContentCodec<GroupMembershipChanges> {
+
+    override fun encode(content: GroupMembershipChanges): EncodedContent {
+        val gson = GsonBuilder().create()
+        return EncodedContent.newBuilder().also {
+            it.type = ContentTypeGroupMembershipChange
+            it.content = gson.toJson(content).toByteStringUtf8()
+        }.build()
+    }
+
+    override fun decode(content: EncodedContent): GroupMembershipChanges {
+        val gson = GsonBuilder().create()
+        return gson.fromJson(content.content.toStringUtf8(), GroupMembershipChanges::class.java)
+    }
+
+    override fun fallback(content: GroupMembershipChanges): String? {
+        return null
+    }
+}