[routing-manager] support Advertising PIO (AP) flag in published route (#9274)

This commit adds support for the Advertising PIO (AP) flag in Network
Data `ExternalRoute` entries. It also updates the `RoutingManager`
class to set the AP flag on its published Network Data route when the
local on-link prefix is included as PIO in emitted RA messages, which
is equivalent to when the local on-link prefix is being published,
advertised, or deprecated.

This commit also updates the `test_routing_manager` unit test to
validate the AP flag in the published route under different
scenarios.

tests/unit/test_network_data.cpp

@@ -134,6 +134,7 @@ void TestNetworkDataIterator(void)
                 false,  // mNat64
                 false,  // mStable
                 false,  // mNextHopIsThisDevice
+                false,  // mAdvPio
             },
             {
                 {
@@ -146,6 +147,7 @@ void TestNetworkDataIterator(void)
                 false,  // mNat64
                 true,   // mStable
                 false,  // mNextHopIsThisDevice
+                false,  // mAdvPio
             },
         };