Capital gains distribution support in activities and corporate actions

Broker API

Adds capital gains distribution detail to account activities.

• New CGDActivityV2 schema, surfaced as a variant of the activity detail payload on the activity event endpoints.
• The ActivitySubType documentation now maps the CGD activity type to its sub-types: LTCG (Long-Term Capital Gain) and STCG (Short-Term Capital Gain).
• The ActivityType descriptions for CGD, DIVCGL, and DIVCGS are reworded to "Capital Gains".

Affected Endpoints

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

Market Data API

Adds capital_gains_distribution as a corporate action type.

• GET /v1/corporate-actions accepts capital_gains_distribution in types and returns a new capital_gains_distributions array, described by the new capital_gains_distribution schema.
• The corporate actions SSE stream adds the capital_gains_distribution_corporateaction_event value to event_type, with the new ca_event_capital_gains_distribution payload schema.

Affected Endpoints

• GET /v1/corporate-actions
• GET /v1beta1/events/corporate-actions