New "asset" object on Activities V2 events

Adds an optional asset object to Activities V2 events, carrying the instrument the activity relates to.

• id and symbol identify the asset; for options activities they describe the option contract, not the underlying.
• cusip and isin are included where available, subject to licensing.
• Existing trade detail fields are unchanged and continue to be returned.

Broker API

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

Trading API

• GET /v2beta1/events/activities