[management] Add option to disable default all-to-all policy (#3970)

This PR introduces a new configuration option `DisableDefaultPolicy` that prevents the creation of the default all-to-all policy when new accounts are created. This is useful for automation scenarios where explicit policies are preferred.
### Key Changes:
- Added DisableDefaultPolicy flag to the management server config
- Modified account creation logic to respect this flag
- Updated all test cases to explicitly pass the flag (defaulting to false to maintain backward compatibility)
- Propagated the flag through the account manager initialization chain

### Testing:

- Verified default behavior remains unchanged when flag is false
- Confirmed no default policy is created when flag is true
- All existing tests pass with the new parameter

client/cmd/testutil_test.go

@@ -103,7 +103,7 @@ func startManagement(t *testing.T, config *types.Config, testFile string) (*grpc
 		Return(&types.Settings{}, nil).
 		AnyTimes()
 
-	accountManager, err := mgmt.BuildManager(context.Background(), store, peersUpdateManager, nil, "", "netbird.selfhosted", eventStore, nil, false, iv, metrics, port_forwarding.NewControllerMock(), settingsMockManager, permissionsManagerMock)
+	accountManager, err := mgmt.BuildManager(context.Background(), store, peersUpdateManager, nil, "", "netbird.selfhosted", eventStore, nil, false, iv, metrics, port_forwarding.NewControllerMock(), settingsMockManager, permissionsManagerMock, false)
 	if err != nil {
 		t.Fatal(err)
 	}