Add load_from_json and save_to_json methods to SingleTableMetadata

[amontanez24]

Problem Description

As a user, it would be useful if I could create an instance of the SingleTableMetadata class from a json file. It would also be useful if I could save an existing instance to a json file.

Expected behavior

• Add a load_from_json method
  • Parameters:
    • filepath: string that is the path to json file
  • Errors:
    • If the json file doesn't contain the SCHEMA_VERSION attribute, then raise the following error

    Error: This metadata file is incompatible with the SingleTableMetadata class and version.

>>> metadata = SingleTableMetadata.load_from_json(filepath='project/metadata.json')

• Add a save_to_json method
  • Add SCHEMA_VERSION to dict privately
  • Parameters:
    • filepath: string that is the path to save the file to
  • Errors:
    • If there is already a file in the path location, raise the following error

    Error: A file named 'metadata.json' already exists in this folder. Please specify a different filename.

>>> metadata.save_to_json(filepath='project/metadata.json')