OAS3: Offer IANA-registered authentication schemes in content assist

[tedepstein]

In the Security Scheme Object, we don't currently provide any help in content assist for the scheme property.

According to the OpenAPI spec, scheme should be:

The name of the HTTP Authorization scheme to be used in the Authorization header as defined in RFC7235.

We might not want to validate this, since the registry is subject to change. But we could take current values and make them available in code assist.