Make spec kinds optional

### Description

A `model` / `aggregate` / `transformed_feature` can be defined without using an `estimator` / `aggregator` / `transformer`. If so, Cortex doesn't validate types.

### Motivation

Reduce the amount of required configuration

### Additional Context

* e.g. `estimator` has a key `path`, and `model` has keys `estimator` and `estimator_path` (only one of them may be defined)
