Allow coder modules to automatically install devcontainer customizations

[mafredri]

Once coder/envbuilder#212 and coder/envbuilder#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.