Read from docker credential helpers when fetching Features/Templates

[joshspicer]

We should also support Docker credential helpers, these are more common in local setups. I recently figured out how to use them for the Dev Containers extension. The code will need to be adapted, it implements a credential helper proxy: https://github.com/microsoft/vscode-remote-containers/blob/main/src/common/dockerCredentialHelper.ts

Originally posted by @chrmarti in #341 (comment)

[samruddhikhandale]

Reported in devcontainers/features#376

It seems like docker's credential helper is no longer injecting the credsStore entry into the .docker/config.json

I think this task is still valid (to support Docker's credential helper) - but at least by default it is no longer getting injected into their config.json

[chrmarti]

Fixed with #460.