Update external OpenID Provider API to reflect updated design

### Description

In previous version we were just using the add_openid_provider endpoint, but in preparation for handling multiple providers we've introduced a dedicated modify endpoint.
Both of them are largely the same at the moment which does not fully reflect the current logic:
- ADD should not reference a current provider
- ADD should return an error if a provider already exists
- PUT uses a provider kind in the query
- this is not necessary if there's only one active provider
- this will be incorrect if we ever support multiple providers since the query parameter is the provider kind and we'll have to support multiple providers of a given kind
- we should add tests that the current single-provider workflow is reflected in the API