ot::NetworkData::BorderRouterFilter enum

Represents the entry filter used when searching for RLOC16 of border routers or servers in the Network Data. Regarding `kBrProvidingExternalIpConn`, a border router 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).