feat: support setting extensions list in the cluster template

Fixes: #45

Introduced new resource type `ExtensionsConfiguration` that allows
setting machine extensions list.

`SchematicConfiguration` is now readonly and is created by
`SchematicConfigurationController` from `ExtensionsConfiguration`
resource. It also ensures that schematic exists in the image factory by
calling the API.

This change is required to simplify the flow in the cluster templates
(no need to call `CreateSchematic` for each resource).

Export command support added as well.
Added cleanup hooks for the `ExtensionsConfiguration` for machine set, machine and cluster levels.

Changed the resource format to use `labels` instead of `target`. Now
it's the same as for config patches, except it doesn't merge several
resources, but gets the first one.

Signed-off-by: Artem Chernyshev <[email protected]>

client/api/omni/specs/omni.proto

@@ -138,6 +138,10 @@ message MachineStatusSpec {
 
     // Extensions is the list of extensions installed on the machine.
     repeated string extensions = 3;
+
+    // InitialSchematic is the one first schematic the machine was using when it was connected to Omni for the first time.
+    // Omni will always reset schematic configuration to this default value when no explicit schematic configuration is set.
+    string initial_schematic = 4;
   }
 
   message MaintenanceConfig {
@@ -932,14 +936,22 @@ message TalosExtensionsSpec {
 
 // SchematicConfigurationSpec is the desired Image Factory schematic for a machine, machine set or a cluster.
 message SchematicConfigurationSpec {
-  enum Target {
+  string schematic_id = 1;
+}
+
+// ExtensionsConfigurationSpec is the desired list of extensions to be installed on the machine or the set of machines.
+message ExtensionsConfigurationSpec {
+  repeated string extensions = 1;
+}
+
+// ExtensionsConfigurationStatusSpec is the status of the generated schematic.
+message ExtensionsConfigurationStatusSpec {
+  enum Phase {
     Unknown = 0;
-    ClusterMachine = 1;
-    MachineSet = 2;
-    Cluster = 3;
+    Ready = 1;
+    Failed = 2;
   }
 
-  string schematic_id = 1;
-
-  Target target = 2;
+  Phase phase = 1;
+  string error = 2;
 }