New REO (Reorganization) activity type

Adds REO to the ActivityType enum. Reorganization events now appear in account activities and the activity event stream with this type.

• Adds the REO value to the activity_type enum.
• Adds REO activity sub-types on activity_sub_type: REOS (stock only), REOC (cash only), and REOSC (stock and cash).
• Adds a REO activity detail variant to the V2 activity event stream.

Broker API

• GET /v1/accounts/activities
• GET /v1/accounts/activities/{activity_type}
• GET /v2beta1/accounts/{account_id}/events/activities/{event_id}
• GET /v2beta1/events/activities