New corporate-action activity types and DIVNRA fields

Adds new voluntary corporate-action activity types to the V2 activity feed and enriches non-resident alien dividend activities.

• New activity types for tender offers, exchange offers, rights subscription elections, and warrant exercise elections.
• Adds cusip, symbol, and parent_id to non-resident alien dividend (DIVNRA) activities.
• ca_id is no longer required on corporate-action activities.

Broker API

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

Trading API

• GET /v2beta1/events/activities