Add new worklow: validate automation schema

Using yamale, we're now ensuring the automation file schema is correct:
- check for required data
- check for data type
- check overall structure of the file

.ci/automation-schema.yaml

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+# This schema is used to validate the automation files.
+# Be sure to update this file if ou add new parameters,
+# or change names or types in the existing files!
+# We are using yamale to validate files
+# https://github.com/23andMe/Yamale
+---
+vas: map(include('_architecture'), key=str())
+
+---
+# Define various nested types
+_values:
+  name: str()
+  src_file: str()
+---
+_hook:
+  name: str()
+  type: str()
+  source: str()
+  inventory: str(required=False)
+---
+_stage:
+  path: str()
+  wait_conditions: list(min=1)
+  values: list(include('_values'), min=1)
+  build_output: str()
+  pre_stage_run: list(include('_hook'), required=False)
+  post_stage_run: list(include('_hook'), required=False)
+---
+_architecture:
+  stages: list(include('_stage'), min=1)

.github/workflows/automation-schema.yaml

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+---
+name: Validate automation schema
+on:  # yamllint disable-line rule:truthy
+  pull_request:
+    branches:
+      - main
+      - stable
+    paths:
+      - ".ci/automation-schema.yaml"
+      - "automation/vars/*.yaml"
+      - "automation/vars/*.yml"
+
+jobs:
+  yamale:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.10'
+
+      - name: Install yaml-lint
+        run: pip install yamale
+
+      - name: Run yamale
+        run: yamale -s .ci/automation-schema.yaml automation/vars/

automation/vars/README.md

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+# Automation files
+
+## Schema validation
+The files are tested using `yamale`. Be sure to update
+the `.ci/automation-schema.yaml` file at the root of this
+project if you add a new key, or modify a type!
+
+More information about [yamale](https://github.com/23andMe/Yamale).
+
+## Manually check schema
+```Bash
+$ pip install yamale
+$ yamale -s .ci/automation-schema.yaml automation/vars
+```