New MEM activity detail type on Activity V2 events

Adds a new MEMActivityV2 variant to the ActivityV2DetailNTA schema, so MEM activities now appear in Activity V2 event payloads. The new detail object includes a required transfer_id (UUID).

Broker API

• GET /v2beta1/accounts/{account_id}/events/activities/{event_id}
• GET /v2beta1/events/activities

Trading API

• GET /v2beta1/events/activities