customizable db locking during migration (#2063)

* customizable db locking during migration

* Update sqlx-core/src/migrate/migrator.rs

Co-authored-by: Austin Bonander <austin.bonander@gmail.com>

* [migrator.rs] cargo fmt

* fix Migrator 'locking' param doctest fail

Co-authored-by: Austin Bonander <austin.bonander@gmail.com>

sqlx-macros/src/migrate.rs

@@ -146,6 +146,7 @@ pub(crate) fn expand_migrator(path: &Path) -> crate::Result<TokenStream> {
                 #(#migrations),*
             ]),
             ignore_missing: false,
+            locking: true,
         }
     })
 }