refactor: make subject sets and subject IDs unambiguous

[zepatrik]

Related issue(s)

closes #708

Checklist

• I have read the contributing guidelines.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

---

BREAKING CHANGES: This patch changes the payload of the REST API. The gRPC API is not affected. The parameter subject was previously an encoded string. With this change clients have to explicitly use either subject_id or (subject_set.namespace and subject_set.object and subject_set.relation). The same is true for REST responses returned by Keto. An error with a hint will be returned if subject is still used.

[aeneasr]

Looks very good high level already