New identifier fields on options activity events

Adds symbol, contract_symbol, and cusip to the common options activity schema, so options activity events now carry full instrument identifiers.

• New OPCSHActivityV2 activity detail schema, added to the activity detail oneOf.

Broker API

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

Trading API

• GET /v2beta1/events/activities