Adds "FI" (Fixed Income Interest) activity sub-type

Adds support for Fixed Income Interest corporate action activity:

• A new FI value under the INT group of the activity_sub_type classification, representing a coupon payment or accrued interest on call.
• A new fixed income interest activity variant in the NTA activity event stream, so these events are now emitted over SSE.

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