New "interest_type", "order_id", and "parent_id" on fixed income interest activities

Adds fields to the fixed income interest (INT/FI) activity schema so coupon and accrued interest can be distinguished and linked to their originating fill:

• A required interest_type field (coupon or accrued).
• Optional order_id and parent_id on trade-accrued interest so the interest activity can be linked to the fill.
• A new FixedIncomeInterestType enum.
• Documents the three variants: coupon, accrued-on-call, and trade accrued.

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