Allow specifying subdirectory to open when port forwarding detected

[heaths]

We have port forwarding set up to be able to browse our static site once generated; however, because we have a baseurl set in our Jekyll _config.yml the browser reports 404. We would like to be able to customize this to open a subdirectory of the site e.g., https://127.0.0.1:4000/azure-sdk.

One idea would be to add this to portAttributes or even otherPortAttributes, which shares the same schema e.g.,

{
  "forwardPorts": [4000],
  "portAttributes": {
    "path": "/azure-sdk"
  }
}

This path could be relative or absolute since it resolves to the site root either way.

[heaths]

I'd be happy to work on this myself, but I don't see any source or mention of it.

[bamurtaugh]

Thanks for also opening in https://github.com/devcontainers/spec/issues/413! I think the spec is a perfect spot to track this, so I'll close this issue in favor of that one.