[proposal] add support to custom headers when using HTTPS reference to a feature

It is currently possible to reference features using https URIs, I think it would be great if headers were passed when the download request is sent.

As the "content" of the feature is already used to define options, an environment variable defining the headers sent with the request could be used. For instance a environment variable declare as this:
```
FEATURE_URI_HEADERS = { : }
```
The tool would parse the environment variable and add corresponding headers. This way, headers used for authentication could be sent while trying to download the tar.gz containing the feature.

PS: I post this proposal as a development of this issue on CLI: https://github.com/devcontainers/cli/issues/531#issuecomment-1574443082