Currently when trying to configure the optionals via configuration-as-code like this:

securityRealm:
    mixing:
       allowsSignup: false
       enableCaptcha: false
       priority: true
       optionals:
            github:
                githubWebUri: "https://github.com"
                githubApiUri: "https://api.github.com"

                ...

Causes an error because `optionals` is not exposed:
 

io.jenkins.plugins.casc.ConfiguratorException: Invalid configuration elements for type class hudson.security.MixingSecurityRealm : optionals. Available attributes : allowsSignup, captchaSupport, enableCaptcha, priority

I suspect the property would need to be exposed as described here:
https://github.com/jenkinsci/configuration-as-code-plugin/blob/master/docs/PLUGINS.md