Downgrades the db specified in the connection config to a schema version.

If downgrade_to_schema_version is greater or equals to zero and less than the current library's schema version, it runs a downgrade transaction to revert the db schema and migrate the data. The update is transactional, and any failure will cause a full rollback of the downgrade. Once the downgrade is done, the user needs to use the older version of the library to connect to the database.

config a proto.ConnectionConfig having the connection params.
downgrade_to_schema_version downgrades the given database to a specific version. For v0.13.2 release, the schema_version is 0. For 0.14.0 and 0.15.0 release, the schema_version is 4. More details are described in g3doc/get_start#upgrade_mlmd_library

errors.InvalidArgumentError if the downgrade_to_schema_version is not given or it is negative or greater than the library version.
RuntimeError if the downgrade is not finished, return detailed error.