Use the "real" signature of the foreign procedure in `ExecFpi` op

I can't help but feel it would be better if we could somehow use the "real" signature of the foreign procedure somehow, probably by storing the type signature in an attribute and making the operands/results `AnyType` - but I see that more as a future improvement - this will do for now I think.

_Originally posted by @bitwalker in https://github.com/0xMiden/compiler/pull/1113#discussion_r3400376854_

查看 ExecFpi 操作的實作以及編譯器中外部程序的處理。該問題建議將型別簽章儲存在屬性中，並將運算元/結果更改為 AnyType。首先檢查 PR #1113 的討論以及目前圍繞外部程序的程式碼，以了解上下文。

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