Allow portal admins to include custom claims in the OIDC ID Token produced by the userinfo endpoint

Someone is going to need this feature before long.

Additional user attributes in the portal may be included in the ID Token as custom claims.  Use the Spring property `org.apereo.portal.security.oauth.IdTokenFactory.customClaims` to specify which additional attributes to include. The claim name will always be the same as the attribute name. The JSON type of a custom claim will be inferred from it's value.