feat: terragrunt support (#7653)

lib/manager/terragrunt/__fixtures__/2.hcl

lib/manager/terragrunt/__snapshots__/extract.spec.ts.snap

lib/manager/terragrunt/extract.spec.ts

lib/manager/terragrunt/extract.ts

lib/manager/terragrunt/index.ts

lib/manager/terragrunt/modules.ts

lib/manager/terragrunt/providers.ts

lib/manager/terragrunt/readme.md

lib/manager/terragrunt/util.spec.ts

lib/manager/terragrunt/util.ts