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-postgres/src/connection/mod.rs

@@ -213,3 +213,13 @@ impl Connection for PgConnection {
         !self.stream.write_buffer().is_empty()
     }
 }
+
+// Implement `AsMut<Self>` so that `PgConnection` can be wrapped in
+// a `PgAdvisoryLockGuard`.
+//
+// See: https://github.com/launchbadge/sqlx/issues/2520
+impl AsMut<PgConnection> for PgConnection {
+    fn as_mut(&mut self) -> &mut PgConnection {
+        self
+    }
+}