Express segments to establish end-to-end segment routing path

Express segments can be used to establish end-to-end TE paths between interconnected TE networks. Express segments (also known as virtual TE links) are generated dynamically through policies matching the underlay LSPs. Express segments and the corresponding abstracted topology (required by RFC7926) is generated with policies. To apply a policy, include the policy policy-name statement at the [edit protocols express-segment traffic-engineering] hierarchy level. To configure express segment, include the express-segment statement under the [edit protocols] hierarchy level.