The provider config has changed a little bit in go-oidc. It is more
complete and now throws errors when unmarshaling provider configs
that are missing required fields (as defined by the OpenID Connect
Discovery spec).
Update the oidc plugin to use the new type.
Also add related new flags to apiserver:
"--oidc-issuer-url", "--oidc-client-id", "--oidc-ca-file", "--oidc-username-claim",
to enable OpenID Connect authentication.
