Add XIP-31: Message History request system

XIPs/assets/xip-31/message-backup-providers-flow.mmd

@@ -0,0 +1,15 @@
+sequenceDiagram
+    Participant B as Backup Requester
+    Participant S as Backup Storage Provider
+    Participant N as XMTP Network
+    Participant M as Message Backup Provider
+    B->>N: Send a MessageHistoryBackupRequest to each potential Message Backup Provider
+    M->>N: Check for MessageHistoryBackupRequest
+    N-->>M: Receive MessageHistoryBackupRequest
+    M->>S: Upload backup file
+    M->>N: Send MessageHistoryBackupResponse
+    B->>N: Check for MessageHistoryBackupResponse
+    N-->>B: Receive response
+    B->>S: Request file from URL in response
+    S-->>B: Receive file
+    B-->>B: Decrypt and load file into database

XIPs/assets/xip-31/message-history-actors.mmd

@@ -0,0 +1,19 @@
+classDiagram
+class R["Backup Requester"]{
+    The new installation requesting missing messages
+    Responsible for selecting a Backup Storage Provider and obtaining authorization
+    Creates MessageHistoryBackupRequests
+    Reads MessageHistoryBackupResponses
+}
+class M["Message Backup Provider"]{
+    An existing installation that has access to message history
+    Reads MessageHistoryBackupRequests
+    Creates MessageHistoryBackupResponses
+    Uploads Message Backup Files to the pre-selected storage provider
+    Responsible for generating one-time encryption keys
+}
+class S["Backup Storage Provider"]{
+    A cloud service with durable storage
+    Stores encrypted backups for a few days
+    Responsible for authenticating requests to reduce abuse
+}