design: Allow implement layer in native way for bindings

[Xuanwo]

To implement a layer in python or nodejs, we should build a PythonLayer or NodejsLayer first.

Let's take Python as an example. First, we will implement Layer for PythonLayer and then call the native function inside it. Bindings can provide their own layer interface based on their specific needs. For instance, a Python binding can expose a layer class that includes functions such as pre_stat, post_stat, pre_read, and post_read.

So the code will be like:

impl Layer for PythonLayer {
    async fn stat(&self, path: &str, op: OpStat) -> Result<Metadata> {
        call_python(pre_stat(path, &op))
        self.inner.stat()
        call_python(post_stat(path, &op))
    }
}

Some design decision:

• binding can define their own Layer class or interface.
• binding should build their own layers, and should not expose rust layer directly (especially for logging).
• binding can reuse existing layer by wrapping a new class (like RetryLayer).

[Zheaoli]

I think we can impl this by allowing add a binding layer in each binding language.(like Python/Java)

This is very important for using language binding in production environment.