OIDC `_security/oidc/prepare` and `_security/oidc/authenticate` APIs should return realm name used for authentication

[azasypkin]

Currently responses of _security/oidc/prepare and _security/oidc/authenticate don't include realm name. That means that if consumer doesn't provide an optional realm parameter they don't know what realm was used to perform authentication.

It's not critical, but it'd help Kibana to properly support IdP initiated logins when multiple OIDC realms is enabled.

Related to: #52053

/cc @jkakavas

[elasticmachine]

Pinging @elastic/es-security (:Security/Authentication)