Add support for Docker credential helpers

[ldx]

This PR adds the ability to use existing Docker credential helpers (like docker-credential-ecr-login, docker-credential-gcr, etc.) for registry authentication. This enables Keel to leverage the same credential management tools that users already have configured in their Docker environments, instead of re-implementing registry credential helpers in Keel itself.

I used the same helper registration method as in the aws and gcr helpers. The only input the new helper needs is the name of an executable that implements the Docker credential protocol (registry name is supplied via stdin, and a command line argument get is added when executing the program). The executable name is passed in via the DOCKER_CREDENTIALS_HELPER environment variable (and can be just the name of the executable without the full path).

The main use case for me was adding Azure ACR support without hacks or implementing an ACR-specific extension, and leveraging https://github.com/chrismellard/docker-credential-acr-env instead, together with Keel polling the registry.