feat: Add support for gitlab-ci include statements (#3946)

lib/config/definitions.js

lib/datasource/gitlab/index.js

lib/manager/gitlabci-include/extract.js

lib/manager/gitlabci-include/index.js

lib/manager/gitlabci-include/update.js

lib/manager/index.js

package.json

renovate-schema.json

test/config/__snapshots__/validation.spec.js.snap

test/datasource/__snapshots__/gitlab.spec.js.snap

test/datasource/gitlab.spec.js

...nager/gitlabci-include/__snapshots__/extract.spec.js.snap

test/manager/gitlabci-include/_fixtures/gitlab-ci.yaml

test/manager/gitlabci-include/extract.spec.js

test/manager/gitlabci-include/update.spec.js

...rkers/repository/extract/__snapshots__/index.spec.js.snap

website/docs/configuration-options.md

yarn.lock