Impl AsMut for advisory lock types (#2520) (#2554)

The documentation for `PgAdvisoryLockGuard` lists a set of types
that should be able to be passed to it, but when actually trying
to do so, compilation would fail due to missing `AsMut` trait
implementations for those types. This commit adds the missing
`AsMut` impls so that `Transaction` and `PgConnection` can be used
as type parameters to `PgAdvisoryLockGuard`, as expected.

For reference: https://github.com/launchbadge/sqlx/issues/2520

sqlx-core/src/transaction.rs

@@ -197,6 +197,16 @@ where
     }
 }
 
+// Implement `AsMut<DB::Connection>` so `Transaction` can be given to a
+// `PgAdvisoryLockGuard`.
+//
+// See: https://github.com/launchbadge/sqlx/issues/2520
+impl<'c, DB: Database> AsMut<DB::Connection> for Transaction<'c, DB> {
+    fn as_mut(&mut self) -> &mut DB::Connection {
+        &mut self.connection
+    }
+}
+
 impl<'c, 't, DB: Database> crate::acquire::Acquire<'t> for &'t mut Transaction<'c, DB> {
     type Database = DB;