chip_data_model.gni: add option for custom-implemented clusters (#22042)

* chip_data_model.gni: add option for custom-implemented clusters

Add `zap_clusters_with_custom_implementation` option to `chip_data_model()` template, which allows to list clusters that need to be available in a device, but don't use the standard implementation in `src/app/clusters/<clustername>/<clustername>.cpp`.

# Usage
To exclude a particular cluster's standard implementation in favor of an application level implementation:
- in the BUILD.gn file for the application specific `.zap` file
- in the instantiation of the `chip_data_model` template
- add a variable `zap_clusters_with_custom_implementation`
- assign it a list of strings with cluster names, corresponding to directories in the `src/app/clusters` directory.
- this causes the cluster implementation file(s) not to be included in the build, allowing re-implementation at the application level.

Example, excluding standard implementation of level-control:
```
chip_data_model("zap") {
  zap_file = "myproject.zap"
  zap_clusters_with_custom_implementation = [ "level-control" ]
  zap_pregenerated_dir = "//zap-generated"
}
```

# Background
Some clusters, for example Level Control, are implemented with the assumption of direct low-level access to the device hardware. For bridged devices, the actual interface might be much more high level, as in my case where I have lights which are fully capable of performing parametrized smooth transitions but are NOT capable of receiving output changes in millisecond intervals. In such cases, a cluster might need application-specific re-implementation.

This changeset allows, from the application's ZAP BUILD.gn, to exclude the standard cluster implementation file, allowing re-implementation of the needed ember callbacks in a application specific file outside the connectedhomeip repo.

* Include suggestions from @bzbarsky-apple

src/app/chip_data_model.gni

@@ -134,6 +134,7 @@ template("chip_data_model") {
                            "*",
                            [
                              "zap_pregenerated_dir",
+                             "zap_clusters_with_custom_implementation",
                              "zap_file",
                              "is_server",
                            ])
@@ -188,8 +189,22 @@ template("chip_data_model") {
                                      [ invoker.zap_file ])
     }
 
+    _custom_impl_clusters = []
+    if (defined(invoker.zap_clusters_with_custom_implementation)) {
+      _custom_impl_clusters = invoker.zap_clusters_with_custom_implementation
+    }
+
     foreach(cluster, _cluster_sources) {
-      if (cluster == "door-lock-server") {
+      _custom_impl = false
+      foreach(ci, _custom_impl_clusters) {
+        if (cluster == ci) {
+          _custom_impl = true
+        }
+      }
+
+      if (_custom_impl) {
+        # do not include any sources, we have a custom implementation for this cluster
+      } else if (cluster == "door-lock-server") {
         sources += [
           "${_app_root}/clusters/${cluster}/door-lock-server-callback.cpp",
           "${_app_root}/clusters/${cluster}/door-lock-server.cpp",