New ROC (Return of Capital) dividend activity sub-type

Adds Return of Capital (ROC) as a cash-dividend activity sub-type.

• ROC is now a documented value of activity_sub_type under the DIV activity type.
• The cash-dividend activity schema now covers both Cash Dividend and Return of Capital, and its share-quantity field describes shares entitled to receive the dividend or return of capital.
• Boolean flags on the cash-dividend and substitute-payment activity schemas are now typed as strings with the values true and false, matching what the API returns.

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

Trading API

• GET /v2/account/activities
• GET /v2/account/activities/{activity_type}
• GET /v2beta1/events/activities