Authentication helper interface is incorrect

[jklukas]

@ianoc added support for authentication helper scripts (started in #343) but we don't document the intended interface for these scripts.

I assume that we are intending to follow the docker credential helper interface as described in https://github.com/docker/docker-credential-helpers?tab=readme-ov-file#development:

A credential helper can be any program that can read values from the standard input. We use the first argument in the command line to differentiate the kind of command to execute

The code here, however, is currently as follows:

rules_minidock_tools/src/registry/http/http_cli/authentication_flow.rs

Line 97 in 351c61c

.write_all(format!("GET {}\n", &auth_fail.service).as_bytes())

There are two separate problems here:

• The current code pipes in "GET" as the first element on stdin, but the credential helper interface expects this as a command line argument
• The current code pipes in "service" as the second element on stdin, which is taken from the previously seen auth challenge, which could be any arbitrary string as decided by the registry; per the docker credential helper spec, this is expected to be the server url

Should we update this to follow the broader credential helper spec, and document that as the interface here?