UI: Support "Open in VSCode" for devcontainers

[johnstcn]

Update the Open in VSCode button URL to construct a vscode-remote://dev-container URL to allow users to jump directly to the devcontainer inside VSCode.

This is an undocumented feature, but some enterprising folks have figured out how to do this: https://github.com/felipecrs/dotfiles/blob/master/home/dot_local/bin/executable_code#L113

More information on this URL format is available here: https://blog.lohr.dev/launching-dev-containers

[DanielleMaywood]

Just wanted to note down my findings so I don't lose them:

vscode://vscode-remote/dev-container+$DEVCONTAINER@ssh-remote+$HOST/workspaces/terraform-provider-coder

Where $DEVCONTAINER is defined as the base64 encoding of

{
    "hostPath": "/home/coder/terraform-provider-coder",
    "configFile":{
        "path": "/home/coder/terraform-provider-coder/.devcontainer/devcontainer.json",
        "scheme": "vscode-remote", 
        "authority": "ssh-remote+$HOST"
    }
}

And $HOST is the remote.

[DanielleMaywood]

I have upgraded the complexity from II to III as this also requires a change to the vscode extension, meaning it's a larger change than originally expected.

[DanielleMaywood]

Looks like my previous findings goes down the wrong path. When using the URI vscode://vscode-remote/dev-container+ it starts a dev container with the given parameters, instead of connecting to an existing dev container.

What we instead want is a URI in the form

vscode://vscode-remote/attached-container+$DEVCONTAINER@ssh-remote+$HOST

Where $DEVCONTAINER is defined as the base64 encoding of

{
    "containerName": "some_container_name"
}

And $HOST is the remote.