ot::NetworkData::Notifier::IsEligibleForRouterRoleUpgradeAsBorderRouter() const method

Indicates whether the device as border router is eligible for router role upgrade request using the status reason `kBorderRouterRequest`. Checks whether device is providing IP connectivity and that there are fewer than two border routers in network data acting as router. Device is considered to provide external IP connectivity if at least one of the below conditions hold: - It has added at least one external route entry. - It has added at least one prefix entry with default-route and on-mesh flags set. - It has added at least one domain prefix (domain and on-mesh flags set). Does not check the current role of device.