[FEATURE REQUEST] Fetching a private repo in github using wit-deps update

I'm trying to include a private repository reference within my wit/deps.toml file. However, upon attempting to update the dependencies using the wit-deps update command, I consistently run into an error message stating "GET request failed (404) URL not found". This indicates that the system is unable to locate the specified URL. My guess was it is a private repository and the `wit-deps` command is not authenticated to access it.

To address this issue and improve functionality, I think it would be highly beneficial to introduce an authentication option for updating wit-deps. This would allow users to securely access and update dependencies from private repositories without encountering the aforementioned error.