OPENTHREAD_CONFIG_BORDER_ROUTING_REACHABILITY_CHECK_ICMP6_ERROR_ENABLE macro

@def OPENTHREAD_CONFIG_BORDER_ROUTING_REACHABILITY_CHECK_ICMP6_ERROR_ENABLE Define to 1 to allow Routing Manager to check for reachability of messages being forwarded by the BR and determine whether to send an ICMPv6 Destination Unreachable error back to the sender. Specifically, if the Border Router (BR) decides to forward a unicast IPv6 message outside the AIL and the message's source address matches a BR-generated ULA OMR prefix (with low preference), and the destination is unreachable using this source address, then an ICMPv6 Destination Unreachable message is sent back to the sender. For example, this situation can occur when a local, non-infrastructure-derived ULA OMR prefix is published alongside a `::/0` route (due to discovered PIO/RIO prefixes by the BR). A Thread mesh device may try to reach addresses beyond the local AIL (e.g., the global internet) using its ULA OMR address as source, which would be unreachable. Alternatively, this functionality may be implemented within the platform layer, in which case this configuration should be disabled. Note that the platform layer is always responsible for implementing generation of "ICMPv6 Destination Unreachable - No Route" messages. This reachability function will only generate "ICMPv6 Destination Unreachable - Communication Administratively Prohibited" messages for specific cases where there may be a default route to the destination but the source address type prohibits usable communication with this destination.