build(ci): added Mergify config (#1016)

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## Description

This PR adds the configuration to properly support
[Mergify](https://mergify.com/) in order to be able to automatically
merge PR requests once the following checks have passed:

- the base branch in set to either `base` or `chains/*` 
- all the PR checks within GitHub are passing 
- the PR has the `automerge` label


## Checklist
- [ ] Ran Linting
- [ ] Targeted PR against correct branch.
- [ ] Linked to Github issue with discussion and accepted design OR link
to spec that describes this work.
- [ ] Wrote unit tests.
- [ ] Added an entry to the `CHANGELOG.md` file.
- [ ] Re-reviewed `Files changed` in the Github PR explorer.

.mergify.yml

@@ -0,0 +1,20 @@
+queue_rules:
+  - name: default
+    conditions:
+      - or:
+        - base=base
+        - base~=chains/
+
+pull_request_rules:
+  - name: Automerge with label automerge and branch protection passing
+    conditions:
+      - "#approved-reviews-by>1"
+      - label=automerge
+    actions:
+      queue:
+        name: default
+        method: squash
+        commit_message_template: >
+          {{ title }} (#{{ number }})
+          
+          {{ body }}