.NET SDK correctly handles Users/Hosts with `api_key_enabled`

[jvanderhoof]

Validate and fix any errors with the following workflows:

• [ ] Loading Policy with users and hosts with API keys disabled shows policy load success/failure, but not API keys
• [ ] Loading Policy updating users and hosts with previously enabled API keys shows policy load success/failure
• [ ] Loading Policy updating users and hosts with previously disabled API keys shows policy load success/failure and generated API keys
• [ ] Attempt to rotate disabled API results in a helpful error message
• [ ] Attempt to authenticate as a user/host with a disabled API key results in a authentication failure

Related to: cyberark/conjur/issues/1359