[Lambda] Layer: Show code source in Lambda Code Source Explorer

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**Tell us about your request**
Enable the Lambda code source explorer to display the code source of layer(s) associated with the function. The code could be read-only but would allow developers to view function definitions and documentation directly within the Lambda console.

**Which service(s) is this request for?**
AWS Lambda

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Currently, when using Lambda layers, there is no direct way to view the source code of functions provided by the layer within the Lambda console. Developers must download .zip file of the layer's source code and read it externaly, which disrupts the workflow and makes debugging or hot fix harder.

**Additional context**
This feature would improve developer productivity by providing better visibility into layer contents.

**Attachments**
N/A

该请求涉及 AWS Lambda Code Source Explorer 以及与某个函数关联的 layers；未指定任何 repository 文件、测试或实现入口点。首先定位 explorer 和 layer-source 处理逻辑，然后直接在 Lambda 控制台中以只读形式查看由 layers 提供的代码和文档，以验证完成后的行为。

由索引模型根据 Issue 内容生成。