Allow generating/accessing a computed `devcontainer.json` inside an envbuilder workspace

To fully support https://github.com/coder/envbuilder/issues/68, we need a way to access all the relevant devcontainer configuration inside a workspace.

This also ties into #128 because currently no devcontainer information is built into the final image. That means that starting a workspace from a cached image won't have any files present from devcontainer features.

There are two options:
1. Have envbuilder store a computed `devcontainer.json` in a well-known location
2. Allow `envbuilder read-configuration` (#210) to download features and inspect their configuration

Option 1 has merit as a simple way of informing coder modules where to look for the configuration. Stored as part of the final image, it would make the configuration immutable until the image is rebuilt.

Option 2 has merit as a drop-in replacement for `@devcontainer/cli` and the behavior can be tweaked by the user. However, this may not guarantee feature immutability.

Option 1 seems like the most straightforward option at this time.

首先阅读相关的 envbuilder read-configuration issue (#210) 以及 #68 和 #128 中的上下文。比较两种提议的方法，然后确定如何在 workspace 中访问相关的 devcontainer 配置，以及从缓存的镜像启动时必须存在什么。

由索引模型根据 Issue 内容生成。