chore: adds GH action for MMI (#23778)

## **Description**

Creates a GH action to automatically notify MMI team members in order to
have immediate visibility of what’s to come and test/review.

Meaning, it will automatically notify us to our extension slack channel,
when a PR with the "team-mmi" tag is merged to develop.

## **Related issues**

Fixes:

## **Manual testing steps**

1. Go to this page...
2.
3.

## **Screenshots/Recordings**

<!-- If applicable, add screenshots and/or recordings to visualize the
before and after of your change. -->

### **Before**

<!-- [screenshots/recordings] -->

### **After**

<!-- [screenshots/recordings] -->

## **Pre-merge author checklist**

- [ ] I’ve followed [MetaMask Coding
Standards](https://github.com/MetaMask/metamask-extension/blob/develop/.github/guidelines/CODING_GUIDELINES.md).
- [ ] I've completed the PR template to the best of my ability
- [ ] I’ve included tests if applicable
- [ ] I’ve documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [ ] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-extension/blob/develop/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

---------

Co-authored-by: David Murdoch <[email protected]>
Co-authored-by: legobeat <[email protected]>

.github/workflows/add-mmi-reviewer-and-notify.yml

@@ -0,0 +1,48 @@
+name: Notify MMI team via Slack
+
+on:
+  pull_request_target:
+    branches:
+      - develop
+    types:
+      - opened
+      - reopened
+      - synchronize
+      - labeled
+
+jobs:
+  process-label:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+      contents: write
+    steps:
+      - name: Notify MMI team via Slack
+        if: contains(github.event.pull_request.labels.*.name, 'team-mmi')
+        uses: slackapi/slack-github-action@007b2c3c751a190b6f0f040e47ed024deaa72844
+        with:
+          status: custom
+          fields: repo,message,commit,author,action
+          payload: |
+            {
+              "text": "A PR with label 'team-mmi' was added and requires review: ${{ github.event.pull_request.html_url }} in ${{ github.repository }}",
+              "attachments": [
+                {
+                  "color": "#2eb886",
+                  "fields": [
+                    {
+                      "title": "Repository",
+                      "value": "${{ github.repository }}",
+                      "short": true
+                    },
+                    {
+                      "title": "PR",
+                      "value": "#${{ github.event.pull_request.number }}",
+                      "short": true
+                    }
+                  ]
+                }
+              ]
+            }
+        env:
+          SLACK_WEBHOOK_URL: ${{ secrets.MMI_LABEL_SLACK_WEBHOOK_URL }}