Change `Pool` to hold a callback instead of a set of grants?

In https://github.com/bytecodealliance/wasmtime/pull/7662, there is discussion that a callback would be more useful than a simple set of allowed addresses. If that's true for Wasmtime, then it's true for cap-std in general, so we should consider migrating cap-std in that direction too.

The current code that maintains a list of allowed addresses could be factored out into a utility that users could use to implement the callback if they need that functionality.

阅读 Wasmtime PR 7662 中的讨论，然后检查 cap-std 当前用于维护允许地址列表的代码。确定 Pool 是否应接受 callback，以及现有的地址列表行为是否应成为可复用的 utility；完成的标准是确定 API 方向和相应的实现范围。

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