feat: support --fake for aerich upgrade (#398)

* feat: support `--fake` for aerich upgrade

* Add `--fake` to downgrade

* tests: check --fake result for aerich upgrade and downgrade

* Update readme

* Fix unittest failed because of `db_field_types` changed

* refactor: improve type hints and document

tests/assets/sqlite_migrate/models.py

@@ -0,0 +1,5 @@
+from tortoise import Model, fields
+
+
+class Foo(Model):
+    name = fields.CharField(max_length=60, db_index=False)