Feature Request: Generate OpenAPI spec for the API to help development of clients

The documentation in `/doc/api/` are good but not detailed enough. For instance;

- It is not clear which parameters will be in the path, which ones will be in the queries, or they should be in the JSON data as properties.
- It is not clear what kind of errors are expected
- Lacking examples to clarify the success and fail scenarios

It would be great if CFSSL provides an OpenAPI spec file along with it so that it would improve the process of writing clients.

Start by reviewing the existing documentation in /doc/api/ and the CFSSL API surface it describes. Produce an OpenAPI spec that distinguishes path, query, and JSON parameters, documents expected errors, and includes success and failure examples for the covered API.

Written by the indexing model from the issue text.