feat: add support for features in registries that require authentication

[derektamsen]

Add support for fetching feature layers from registries that require
authentication. The authentication pattern mimics what is done in other
places in the codebase. It will search the running environment for registry
credentials and use them to authenticate.

To setup authentication follow the same documentation as for pulling
other images from private registries.

fixes #457

[mcharris]

I was able to use your build with success. It was with the Kubernetes (Devcontainer) template (fixing the module code-server of course) and switching the locals to use the image:

locals {
  #devcontainer_builder_image = data.coder_parameter.devcontainer_builder.value
  devcontainer_builder_image = "ghcr.io/derektamsen/envbuilder-preview:latest"
}

[mcharris]

Not sure why this hasn't been reviewed and merge. Includes tests and valid code. Nice work!

[johnstcn]

Nice work!