IS-IS: TWAMP Light: Unidirectional link delay metrics in Extended Reachability TLV

When TWAMP Light is enabled on an IS-IS link, the device measures unidirectional link performance during each measurement cycle and advertises the resulting metrics in IS-IS sub-TLVs. Supported metrics include average, minimum, and maximum unidirectional link delay, as well as unidirectional delay variation. These measurements provide dynamic link-performance information that can be used for path-selection decisions based on observed latency and delay variation rather than relying solely on traditional IGP cost metrics.