OPENTHREAD_CONFIG_BORDER_ROUTING_USE_HEAP_ENABLE macro

@def OPENTHREAD_CONFIG_BORDER_ROUTING_USE_HEAP_ENABLE Define to 1 to allow using heap by Routing Manager. When enabled heap allocated entries will be used to track discovered prefix table contain information about discovered routers and the advertised on-link prefixes on infra link. When disabled pre-allocated pools are used instead where max number of entries are specified by `OPENTHREAD_CONFIG_BORDER_ROUTING_MAX_DISCOVERED_ROUTERS` and `MAX_DISCOVERED_PREFIXES` configurations.