OPENTHREAD_CONFIG_BORDER_ROUTER_REQUEST_ROUTER_ROLE macro

@def OPENTHREAD_CONFIG_BORDER_ROUTER_REQUEST_ROUTER_ROLE Define to 1 to enable mechanism on a Border Router which provides IP connectivity to request router role upgrade. This config is applicable on an `OPENTHREAD_FTD` build and when `OPENTHREAD_CONFIG_BORDER_ROUTER_ENABLE` is also enabled. 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). A Border Router which provides IP connectivity and is acting as a REED is eligible to request a router role upgrade by sending an "Address Solicit" request to leader with status reason `BorderRouterRequest`. This reason is used when the number of active routers in the Thread mesh is above the threshold, and only if the number of existing eligible BRs (determined from the Thread Network Data) that are acting as router is less than two. This mechanism allows up to two eligible Border Routers to request router role upgrade when the number of routers is already above the threshold.