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: {}

Start from the TlsVerification configuration and the userInfo Keycloak backend described in the issue, then trace how TLS settings are deserialized and applied to the HTTP client. Verify the default uses WebPKI trust, while explicit null, none, and internal-PKI configurations retain their intended behavior; add or update relevant tests if the repository provides them.