Enable custom Signer for more flexible Account Abstraction development

Enable more flexible development of Account Abstraction by allowing Starknet Rivet users to set custom Signers.

TODO: Discuss how to do the actual implementation. Should it be some boilerplate template that the user would pull with UI, edit in TS, and then set back using UI? In this case, it would be some hook callback methods for the signer or something else.

Explore possible solutions.

該 issue 是關於在 Starknet Rivet 中為帳戶抽象啟用自訂簽名器。首先探索程式碼庫中現有的簽名器實作，可能在 `src/signer` 或 `src/account` 等目錄中。尋找定義簽名器行為的介面或抽象類別。TODO 提到討論實作，因此首先要理解目前的架構以及簽名器是如何整合的。「完成」將是一個設計提案或一個允許使用者注入自訂簽名器邏輯的掛鉤系統。

由索引模型根據 Issue 內容生成。