New "enabled_assets" field on the account update request

Broker API

Adds the optional enabled_assets array to the account update request body, so enabled asset classes can be modified when updating an account.

• Omitting enabled_assets leaves the account's enabled assets unchanged.
• Updates are additive only: the submitted list must include every already-enabled asset class, and assets cannot be removed.
• Enabling crypto on an equities-only account requires submitting ["us_equity", "crypto"] along with the signed crypto agreement.
• The field uses the same asset class enum as the account creation request and the account response.

Affected Endpoints

• PATCH /v1/accounts/{account_id}