[keycloak-user] Add x5c to be generated as part of the JWK

Hi, In an effort to use keycloak as an IDP with Azure for multi factor authentication, Azure expects the x5c property to be available in the JWK when querying the jwks_uri. The change would simply include the x5c property as part of the returned JWK as described here (https://tools.ietf.org/html/rfc7517#section-4.7). In short, the x5c property contains the certificate chain. The change would involve updating core\src\main\java\org\keycloak\jose\jwk\JWKBuilder.java to generate the certificate chain from the RsaKeyMetadata certificate. Thanks