New "trade_marker" field on rebalancing subscriptions

Broker API

Adds the optional trade_marker field to the rebalancing subscription create request and to the subscription object.

Omitting trade_marker means discretionary. A discretionary subscription is rejected with 422 unless the correspondent's advisor is marked discretionary.
Send unsolicited to create a subscription without that advisor flag.
Subscriptions created before trade markers return trade_marker: null, and their orders stay unmarked.
Adds rebalancing_discretion_not_authorized and rebalancing_invalid_trade_marker error examples, and marks id, account_id, portfolio_id, created_at, and trade_marker as required on the subscription response schema.