Implement /identityproviders endpoint

[jmgasper]

The /identityproviders endpoint is necessary for SSO login by Wipro (and other orgs that don't use the typical username / password flow). This is documented in the Swagger, but doesn't appear to be implemented at all.

v3 logic: https://github.com/appirio-tech/identity-service/blob/bf2428ea6997e115b317eb419c5e2c1c6029cdf5/src/main/java/com/appirio/tech/core/service/identity/resource/IdentityProviderResource.java#L1