Allow coder modules to automatically install devcontainer customizations

Once https://github.com/coder/envbuilder/issues/212 and https://github.com/coder/envbuilder/issues/211 are implemented, we should add support for modules to read the devcontainer configuration and perform relevant actions.

In the case of `code-server`, this may mean installing extensions and applying configuration.

Typically this configuration is in JSON, if need be, we may consider providing the configuration to Coder modules in a more palatable format.