SRv6: Dynamic SID support for BGP

We support dynamic segment identifiers (SIDs) for BGP. To enable dynamic end SID, include the dynamic-end-sid at the [edit protocols isis source-packet-routing srv6 locator locator-name ] hierarchy level. To enable dynamic end x SID, include the dynamic-end-x-sid at the [edit protocols isis interface int-name level level-number srv6-adjacency-segment protected locator locator-name ] hierarchy level.