fix(model): change configuration from struct to string

[xiaofei-du]

Because

• We can safely use string to replace google.protobuf.Struct for now, as google.protobuf.FieldMask just doesn’t work with google.protobuf.Struct at all. Another thing is that we store a JSON struct type in the database as JSONB and its GORM mapping is datatypes.JSON which is nothing but []byte (i.e., a string).

This commit

• update configuration in model and model_instance from google.protobuf.Struct to string

Note:

This PR relies on #71.

[pinglin]

LGTM