Fix: service LocallyRegisteredAsSidecar property is not persisted

When a service is deregistered, we check whever matching services were
registered as sidecar along with it and deregister them as well.
To determine if a service is indeed a sidecar we check the
structs.ServiceNode.LocallyRegisteredAsSidecar property. However, to
avoid interal API leakage, it is excluded from JSON serialization,
meaning it is not persisted to disk either.
When the agent is restarted, this property lost and sidecars are no
longer deregistered along with their parent service.
To fix this, we now specifically save this property in the persisted
service file.

agent/agent.go

@@ -1701,6 +1701,11 @@ type persistedService struct {
 	Token   string
 	Service *structs.NodeService
 	Source  string
+	// whether this service was registered as a sidecar, see structs.NodeService
+	// we store this field here because it is excluded from json serialization
+	// to exclude it from API output, but we need it to properly deregister
+	// persisted sidecars.
+	LocallyRegisteredAsSidecar bool `json:",omitempty"`
 }
 
 // persistService saves a service definition to a JSON file in the data dir
@@ -1709,9 +1714,10 @@ func (a *Agent) persistService(service *structs.NodeService, source configSource
 	svcPath := filepath.Join(a.config.DataDir, servicesDir, svcID.StringHash())
 
 	wrapped := persistedService{
-		Token:   a.State.ServiceToken(service.CompoundServiceID()),
-		Service: service,
-		Source:  source.String(),
+		Token:                      a.State.ServiceToken(service.CompoundServiceID()),
+		Service:                    service,
+		Source:                     source.String(),
+		LocallyRegisteredAsSidecar: service.LocallyRegisteredAsSidecar,
 	}
 	encoded, err := json.Marshal(wrapped)
 	if err != nil {
@@ -3160,6 +3166,10 @@ func (a *Agent) loadServices(conf *config.RuntimeConfig, snap map[structs.CheckI
 				continue
 			}
 		}
+
+		// Restore LocallyRegisteredAsSidecar, see persistedService.persistedService
+		p.Service.LocallyRegisteredAsSidecar = p.LocallyRegisteredAsSidecar
+
 		serviceID := p.Service.CompoundServiceID()
 
 		source, ok := ConfigSourceFromName(p.Source)