Implement json storage (#3189)

Summary:
Pull Request resolved: #3189

Implement two new methods: save_to_json_file and load_from_json_file

Also renamed DBConfig to StorageConfig, made URL optional, and added optional RegistryBundle field.

Reviewed By: lena-kashtelyan

Differential Revision: D67159759

fbshipit-source-id: e79611d2bffc22cdac4e788d7af38271a3d5c486

ax/preview/api/__init__.py

@@ -9,26 +9,26 @@
 from ax.preview.api.client import Client
 from ax.preview.api.configs import (
     ChoiceParameterConfig,
-    DatabaseConfig,
     ExperimentConfig,
     GenerationStrategyConfig,
     OrchestrationConfig,
     ParameterScaling,
     ParameterType,
     RangeParameterConfig,
+    StorageConfig,
 )
 from ax.preview.api.types import TOutcome, TParameterization
 
 __all__ = [
     "Client",
     "ChoiceParameterConfig",
-    "DatabaseConfig",
     "ExperimentConfig",
     "GenerationStrategyConfig",
     "OrchestrationConfig",
     "ParameterScaling",
     "ParameterType",
     "RangeParameterConfig",
+    "StorageConfig",
     "TOutcome",
     "TParameterization",
 ]