Update spec with new auth management endpoints

[telday]

What does this PR do?

Added framework for new potential endpoints to the Conjur API spec.

What ticket does this PR close?

Resolves #[relevant GitHub issues, eg 76] Relates to cyberark/conjur#[Relevant conjur issue number]

Checklists

Change log

• [ ] The CHANGELOG has been updated, or
• [ ] This PR does not include user-facing changes and doesn't require a CHANGELOG update

Test coverage

• [ ] This PR includes new unit and integration tests to go with the code changes, or
• [ ] The changes in this PR do not require tests

Documentation

• [ ] Docs (e.g. READMEs) were updated in this PR, and/or there is a follow-on issue to update docs, or
• [ ] This PR does not require updating any documentation

[jvanderhoof]

@telday this is a great star! I've got a couple of suggestions:

• What do you think about having authenticator specific endpoints? There are some nuances between the various authenticators (ex. there can be only one GCP authenticator per account). Providing authenticator specific endpoints could allow the create endpoint to fully create an authenticator in one step rather than just generate the policy. For example, the authn-k8s/<authenticator-id> endpoint could accept service-account, url, and ca certificate and generate and set the key pair.
• Is it possible to generate a Host (identity) through the API for a particular authenticator? Supporting this would greatly simplify the process of onboarding platform identities into Conjur.