User Info Fetcher: Enable TLS with WebPKI trust by default

Currently, the following configuration leads to the HTTP client connecting without TLS:

apiVersion: opa.stackable.tech/v1alpha1
kind: OpaCluster
metadata:
  name: test-opa
spec:
  clusterConfig:
    userInfo:
      backend:
        keycloak:
          hostname: keycloak.example.com
          clientCredentialsSecret: opa-infofetcher-keycloak-secret
          adminRealm: master
          userRealm: master

And to enable TLS, you have to jump through a few hoops by adding:

          tls:
            verification:
              server:
                caCert:
                  webPki: {}

In this day-in-age, I think it is expected to default to TLS (and the CRA requires secure-by-default).

So I propose that we impl Default for tls:

impl Default for TlsVerification {
    fn default() -> Self {
        Self::Server(TlsServerVerification {
            ca_cert: CaCert::WebPki {},
        })
    }
}

... and explicit steps are to be taken to disable TLS or to ignore verification (or set internal PKI), eg:

          tls: null 

or

          tls:
            verification:
              none: {}

Commencez par la configuration TlsVerification et le backend userInfo de Keycloak décrits dans l’issue, puis suivez la manière dont les paramètres TLS sont désérialisés et appliqués au client HTTP. Vérifiez que la valeur par défaut utilise la confiance WebPKI, tandis que les configurations explicites null, none et internal-PKI conservent leur comportement prévu ; ajoutez ou mettez à jour les tests pertinents si le dépôt en fournit.