Adds `borrow_status` and deprecates `easy_to_borrow` on assets

Adds an optional borrow_status field to the asset schema and deprecates easy_to_borrow.

• borrow_status is a new optional field on asset responses indicating an asset's borrow availability.
• easy_to_borrow is deprecated and will be removed on 2026-09-22. Use borrow_status instead.

Broker API

• GET /v1/assets
• GET /v1/assets/{symbol_or_asset_id}

Trading API

• GET /v2/assets
• GET /v2/assets/{symbol_or_asset_id}