Adding back bin/setup-composer-auth (needs add'l testing)

compose/bin/setup-composer-auth

@@ -0,0 +1,29 @@
+#!/bin/bash
+set -o errexit
+
+MAGENTO_USERNAME_PROP="http-basic.repo.magento.com.username"
+MAGENTO_PASSWORD_PROP="http-basic.repo.magento.com.password"
+hash composer 2>/dev/null && IS_COMPOSER_ON_HOST=true
+
+if [ $IS_COMPOSER_ON_HOST ]; then
+    PUBLIC_KEY="$(composer config --global $MAGENTO_USERNAME_PROP 2>/dev/null)"
+    PRIVATE_KEY="$(composer config --global $MAGENTO_PASSWORD_PROP 2>/dev/null)"
+fi
+
+if [ -z "$PUBLIC_KEY" ] || [ -z "$PRIVATE_KEY" ]; then
+    exec < /dev/tty
+    echo
+    echo "    Authentication required (repo.magento.com, public_key and private_key):"
+    read -r -p "        Username: " PUBLIC_KEY
+    read -r -p "        Password: " PRIVATE_KEY
+    echo
+    exec <&-
+fi
+
+if [ -z "$PUBLIC_KEY" ] || [ -z "$PRIVATE_KEY" ]; then
+    echo "Please setup Composer auth for repo.magento.com to continue." && exit 1
+fi
+
+bin/clinotty composer config --global http-basic.repo.magento.com "$PUBLIC_KEY" "$PRIVATE_KEY"
+
+echo "Composer auth has been setup."