Support BYO credential helpers

[ollypom]

What is the problem you're trying to solve?.
Today Finch only "natively" supports the ecr-login credential helper #462. However there are scenarios where folks may want to use other credential helpers or wrap ecr-login.

Today you can put your own helpers in ~/.finch/cred-helpers and update your DOCKER_CONFIG config.json and everything will be successfully passed into the lima VM, however the BYO credential helper would not be in your path.

$ cat .finch/config.json
{
        "credsStore": "ecr-login-new"
}

$ ls -l .finch/cred-helpers
total 6512
-rwxrwxrwx 1 root root 6668288 Feb  1 13:42 docker-credential-ecr-login
-rwxrwxrwx 1 root root      42 Feb  8 11:24 docker-credential-ecr-login-new

$ finch run 111222333444.dkr.ecr.eu-west-1.amazonaws.com/nginx:latest
WARN[0000] cannot get auth config for authConfigHostname="111222333444.dkr.ecr.eu-west-1.amazonaws.com" (refHostname="111222333444.dkr.ecr.eu-west-1.amazonaws.com")  error="error getting credentials - err: exec: \"docker-credential-ecr-login-new\": executable file not found in $PATH, out: ``"

Today we create a symlink explicitly for the ecr-login provider so that's its in your path.

finch/pkg/config/nerdctl_config_applier.go

Lines 87 to 92 in 52fb8b2

	cmdArr := []string{
	`export DOCKER_CONFIG="$FINCH_DIR"`,
	"[ -L /usr/local/bin/docker-credential-ecr-login ] " +
	`|| sudo ln -s "$FINCH_DIR"/cred-helpers/docker-credential-ecr-login /usr/local/bin/`,
	`[ -L /root/.aws ] || sudo ln -fs "$AWS_DIR" /root/.aws`,
	}

I wonder if we could instead add "$FINCH_DIR"/cred-helpers/ to the users path. Therefore all credentials providers could be picked up 🤔

Describe the feature you'd like
Support BYO credential providers / wrapper scripts.

Additional context
Add any other context or screenshots about the feature request here.