feat: introduce keyupdate messages informing contacts about relay changes

When the published relay list changes, key-contacts are informed with an
unsigned message carrying the re-signed key, encrypted to a chunk of contacts
at a time. It is shaped like a receipt notification naming no message, so that
cores which know nothing about keyupdates trash it as well.

See the src/keyupdate.rs module docs for the design.

src/sql/migrations.rs

@@ -2610,6 +2610,19 @@ UPDATE msgs SET state=24 WHERE state=18; -- Change OutPreparing to OutFailed.
         .await?;
     }
 
+    inc_and_check(&mut migration_version, 164)?;
+    if dbversion < migration_version {
+        // Seed the keyupdate baseline so that upgrading alone sends nothing,
+        // see `keyupdate.rs`.
+        sql.execute_migration(
+            "INSERT OR REPLACE INTO config (keyname, value)
+             SELECT 'keyupdate_baseline', IFNULL(group_concat(addr, ' ' ORDER BY addr), '')
+             FROM transports WHERE is_published=1",
+            migration_version,
+        )
+        .await?;
+    }
+
     let new_version = sql
         .get_raw_config_int(VERSION_CFG)
         .await?

src/sql/migrations/migrations_tests.rs

@@ -8,6 +8,7 @@ use crate::contact::ContactId;
 use crate::contact::Origin;
 use crate::test_utils::TestContext;
 use crate::tools;
+use crate::transport::add_pseudo_transport;
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_clear_config_cache() -> anyhow::Result<()> {
@@ -29,6 +30,29 @@ async fn test_clear_config_cache() -> anyhow::Result<()> {
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_keyupdate_baseline_migration() -> Result<()> {
+    let configured = STOP_MIGRATIONS_AT
+        .scope(163, async move { TestContext::new_alice().await })
+        .await;
+    // An address sorting before the primary pins the seed's ORDER BY,
+    // an unpublished transport pins its filter.
+    add_pseudo_transport(&configured, "aa@example.org").await?;
+    add_pseudo_transport(&configured, "unpublished@example.org").await?;
+    configured
+        .sql
+        .execute(
+            "UPDATE transports SET is_published=0 WHERE addr='unpublished@example.org'",
+            (),
+        )
+        .await?;
+    configured.sql.run_migrations(&configured).await?;
+    let relays = configured.get_config(Config::KeyupdateBaseline).await?;
+    assert_eq!(relays.as_deref(), Some("aa@example.org alice@example.org"));
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_key_contacts_migration_autocrypt() -> Result<()> {
     let t = STOP_MIGRATIONS_AT