nixos/kubernetes: add extraConfig to kubelet config (#338526)

- Kanidm previously had an incorrect systemd service type, causing dependent units with an `after` and `requires` directive to start before `kanidm*` finished startup. The module has now been updated in line with upstream recommendations.

- The kubelet configuration file can now be amended with arbitrary additional content using the `services.kubernetes.kubelet.extraConfig` option.

- To facilitate dependency injection, the `imgui` package now builds a static archive using vcpkg' CMake rules.

  The derivation now installs "impl" headers selectively instead of by a wildcard.

  Use `imgui.src` if you just want to access the unpacked sources.

    // lib.optionalAttrs (cfg.clusterDomain != "")  { clusterDomain = cfg.clusterDomain; }

    // lib.optionalAttrs (cfg.clusterDns != [])     { clusterDNS = cfg.clusterDns; }

    // lib.optionalAttrs (cfg.featureGates != {})   { featureGates = cfg.featureGates; }

    // lib.optionalAttrs (cfg.extraConfig != {})    cfg.extraConfig

  ));

  manifestPath = "kubernetes/manifests";

      type = separatedString " ";

    };

    extraConfig = mkOption {

      description = "Kubernetes kubelet extra configuration file entries.";

      default = {};

      type = attrsOf attrs;

    };

    featureGates = mkOption {

      description = "Attribute set of feature gate";

      default = top.featureGates;