Added a new contextual attribute: performer.activity_token
This field was requested by the Abstract Wikipedia team to be included in the Wikilambda instrument we are migrating to use core interaction events.
Its value corresponds to the activity_session field that is documented in datahub: https://datahub.wikimedia.org/glossaryTerm/urn:li:glossaryTerm:0f6b8770-8db9-4eb5-b205-4225daf5bb1b/Documentation?is_lineage_mode=false
Bug: T358758