Add a `problem_statement` field on `Task`

[cweill]

Task should have problem_statement property which returns a ProblemStatement proto. Several TFX components consume this proto, and therefore we should use it.

A task is essentially a two-tuple of (dataset, problem_statement).

We only need to populate the minimal fields on the proto. A minimalist proto (in text format) would look like:

owner: "nitroml"
tasks {
  type {
    multi_class_classification {
      label: "Contraceptive_method_used"
      n_classes: 3
    }
  }
  name: "openml_cmc"
}