EZP-22743: Missing refresh session link in the root resource of the REST API

JIRA: https://jira.ez.no/browse/EZP-22743

# Description

The session refresh resource was introduced in #753 but it is not referenced in the root resource of the REST API forcing any REST client to have it hardcoded somewhere.

This patch just adds the link to this resource in the root of the REST API v2.

# Tests

manual tests