ci(Mergify): configuration update (#18)

Signed-off-by: Lin Yang <reaver@flomesh.io>

.mergify.yml

@@ -1 +1,39 @@
 
+pull_request_rules:
+  - name: Automatic squash and merge when CI passes and reviews approve
+    conditions:
+      # True if the base branch is main
+      - base=main
+      # check-success are true if the check ran successfully
+      - "check-success=DCO"
+      - "check-success=CodeQL"
+      - "check-success=ShellCheck"
+      - "check-success=Lint"
+      - "check-success=Codegen"
+      - "check-success=Go build"
+      # True if the PR has at least 1 approving reviews
+      - "#approved-reviews-by >= 1"
+      # True if the PR has the auto-squash label
+      - label=auto-squash
+      # True if the PR does not have the wip label
+      - -label=wip
+      # True if the PR does not have the do-not-merge/hold label
+      - -label=do-not-merge/hold
+    actions:
+      merge:
+        # Squashes commits then merges PR
+        method: squash
+
+  - name: Rebase pull request when it's more than 3 commits behind main
+    conditions:
+      - base = main
+      - "#commits-behind >= 3"
+    actions:
+      rebase:
+
+  - name: Add a label auto-squash to the PR automatically
+    conditions: []
+    actions:
+      label:
+        add: 
+          - "auto-squash"