New "tax_rate" and "tax_country" fields on DIVNRA activities

Adds tax_rate and tax_country to the DIVNRAActivityV2 schema, exposing the tax country and withholding rate applied when the activity was booked.

Broker API

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

Trading API

• GET /v2beta1/events/activities