Allow the schema to be passed as part of the context when creating or updating a dataset

Extensions used to have a way of tweaking the schema before calling `package_create` or `package_update` and adding it to the context (this was picked up on `form_to_db_schema_options`). This has now been lost with the latest IDatasetForm refactoring, which ignores the schema in the context and always calls `package_plugin.create_package_schema()` or `package_plugin.update_package_schema()`