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/fake/settings.py

@@ -0,0 +1,22 @@
+import os
+from datetime import date
+
+from tortoise.contrib.test import MEMORY_SQLITE
+
+DB_URL = (
+    _u.replace("\\{\\}", f"aerich_fake_{date.today():%Y%m%d}")
+    if (_u := os.getenv("TEST_DB"))
+    else MEMORY_SQLITE
+)
+DB_URL_SECOND = (DB_URL + "_second") if DB_URL != MEMORY_SQLITE else MEMORY_SQLITE
+
+TORTOISE_ORM = {
+    "connections": {
+        "default": DB_URL.replace(MEMORY_SQLITE, "sqlite://db.sqlite3"),
+        "second": DB_URL_SECOND.replace(MEMORY_SQLITE, "sqlite://db_second.sqlite3"),
+    },
+    "apps": {
+        "models": {"models": ["models", "aerich.models"], "default_connection": "default"},
+        "models_second": {"models": ["models_second"], "default_connection": "second"},
+    },
+}