feat(dns-server): check system prerequisites when init/add machine: DNS port is not in use by another DNS server

internal/machine/api/pb/machine.proto

@@ -8,6 +8,8 @@ import "google/protobuf/empty.proto";
 import "internal/machine/api/pb/common.proto";
 
 service Machine {
+  // CheckPrerequisites verifies if the machine meets all necessary system requirements to participate in the cluster.
+  rpc CheckPrerequisites(google.protobuf.Empty) returns (CheckPrerequisitesResponse);
   rpc InitCluster(InitClusterRequest) returns (InitClusterResponse);
   rpc JoinCluster(JoinClusterRequest) returns (google.protobuf.Empty);
   rpc Token(google.protobuf.Empty) returns (TokenResponse);
@@ -29,6 +31,13 @@ message NetworkConfig {
   bytes public_key = 4;
 }
 
+message CheckPrerequisitesResponse {
+  // Overall status of the checks.
+  bool satisfied = 1;
+  // Error message if not satisfied (empty if all checks pass).
+  string error = 2;
+}
+
 message InitClusterRequest {
   string machineName = 1;
   IPPrefix network = 2;