Add .gitlab-ci.yml with template from https://gitlab.com/drupalspoons/composer-plugin

Closes #3268636

.gitlab-ci.yml

+include:
+  # This remote project centralize our CI "golden path" https://docs.gitlab.com/ee/ci/yaml/#includefile
+  remote: 'https://gitlab.com/drupalspoons/composer-plugin/-/raw/master/templates/.gitlab-ci.yml'
+
+#
+# Below, projects may override anything in the template above.
+#
+
+# Rarely needed override of composer-plugin version. Don't cope this item your project.
+#composer:
+#  variables:
+#    COMPOSER_PLUGIN_CONSTRAINT: "dev-master"