feat(yamlfmt): Add shared CI definition for `yamlfmt`.

* Print a diff of the changes to STDOUT.
* Upload a patch that users can apply to fix the issues.
* Print instructions to download and apply the patch.

Issue: gitlab-com/gl-infra/common-template-copier#6

yamlfmt.md

+# yamlfmt
+
+Runs **[yamlfmt](https://github.com/google/yamlfmt)** to format YAML files uniformly.
+
+## Setup
+
+Simply include `yamlfmt.yml` in your `.gitlab-ci.yml` file:
+
+```yaml
+stages:
+  - validate
+
+include:
+  - project: "gitlab-com/gl-infra/common-ci-tasks"
+    ref: v2.47.0  # renovate:managed
+    file: yamlfmt.yml
+```
+
+## Configuration
+
+The *yamlfmt* job provides very limited configuration options. This is by design.
+
+### Excluding files
+
+To exclude files, create a file named `.yamlfmtignore` at the root of your repository.
+The file uses [gitignore](https://git-scm.com/docs/gitignore) syntax to excempt some files, such as test data, from formatting.

yamlfmt.yml

+spec:
+  inputs:
+    stage:
+      default: validate
+---
+
+yamlfmt:
+  stage: $[[ inputs.stage ]]
+  needs: []
+  image:
+    name: registry.gitlab.com/gitlab-com/gl-infra/common-ci-tasks-images/yamlfmt:${GL_ASDF_YAMLFMT_VERSION}
+    entrypoint: [""]
+  script:
+    - |
+      # Create default config if .yamlfmt.yaml doesn't exist.
+      if [ ! -e .yamlfmt.yaml ]; then
+        cat >.yamlfmt.yaml <<EOF
+      formatter:
+        type: basic
+        retain_line_breaks_single: true
+        scan_folded_as_literal: true
+        drop_merge_tag: true
+        pad_line_comments: 2
+        eof_newline: true
+      EOF
+        if [ -e .yamlfmtignore ]; then
+          echo "gitignore_path: .yamlfmtignore" >>.yamlfmt.yaml
+          echo "gitignore_excludes: true" >>.yamlfmt.yaml
+        fi
+      fi
+      echo "# .yamlfmt.yaml"
+      cat .yamlfmt.yaml
+    - yamlfmt -conf .yamlfmt.yaml .
+    - git diff | tee yamlfmt.patch
+    - |
+      if [ -s yamlfmt.patch ]; then
+        red="$(printf "\e[31;1m")"
+        reset="$(printf "\e[0m")"
+        echo "${red}Some YAML files are not formatted correctly. Run the following command in the repository root to fix:${reset}"
+        echo "${red}# curl ${CI_PROJECT_URL}/-/jobs/${CI_JOB_ID}/artifacts/raw/yamlfmt.patch | patch -p1${reset}"
+        exit 1
+      fi
+  after_script: |
+    cat <<-EOD
+    ----------------------------------------------------------
+    Need help? Documentation on the yamlfmt CI job can be found at:
+    https://gitlab.com/gitlab-com/gl-infra/common-ci-tasks/-/blob/main/yamlfmt.md
+    EOD
+  rules:
+    - if: '$GL_ASDF_YAMLFMT_VERSION == null || $GL_ASDF_YAMLFMT_VERSION == ""'
+      when: 'never'
+    - if: '$CI_PIPELINE_SOURCE == "schedule"'
+      when: 'never'
+    - if: '($CI_PIPELINE_SOURCE == "merge_request_event") || ($CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH)'
+  allow_failure: true
+  artifacts:
+    paths:
+      - yamlfmt.patch
+    expose_as: 'yamlfmt patch'
+    when: on_failure