Restart operational advertisements when we get a new opcert. (#7885)

Per the spec, the commissioning flow goes like this:

1. Various steps.
2. Install a new opcert.
3. Optionally configure and enable a network.
4. Discover each other on the operational network.

This means when the commissionnee gets a new opcert it needs to start
operational advertisement (if it's able to), because it doesn't know
whether the next step on the part of the commissioner will be network
config or operational discovery.

In particular, adding a new opcert adds a new fabric and node id, so
we need to redo our operational advertisements in the case when we are
already on the operational network to advertise the new operational
identity.

src/app/clusters/operational-credentials-server/operational-credentials-server.cpp

@@ -26,6 +26,7 @@
 #include <app/common/gen/attribute-type.h>
 #include <app/common/gen/cluster-id.h>
 #include <app/common/gen/command-id.h>
+#include <app/server/Mdns.h>
 #include <app/server/Server.h>
 #include <app/util/af.h>
 #include <app/util/attribute-storage.h>
@@ -343,6 +344,12 @@ bool emberAfOperationalCredentialsClusterAddOpCertCallback(chip::app::Command *
     VerifyOrExit(admin->SetOperationalCert(OperationalCert) == CHIP_NO_ERROR, status = EMBER_ZCL_STATUS_FAILURE);
     VerifyOrExit(GetGlobalAdminPairingTable().Store(admin->GetAdminId()) == CHIP_NO_ERROR, status = EMBER_ZCL_STATUS_FAILURE);
 
+    // We have a new operational identity and should start advertising it.  We
+    // can't just wait until we get network configuration commands, because we
+    // might be on the operational network already, in which case we are
+    // expected to be live with our new identity at this point.
+    chip::app::Mdns::StartServer();
+
 exit:
     emberAfSendImmediateDefaultResponse(status);
     if (status == EMBER_ZCL_STATUS_FAILURE)