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

[Work Account]

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