Add Github workflow to validate YAML schema

This will allow us to ensure the YAML is well formatted in PRs. Note, we
would like to make `source` a required field, but the YAML file
currently fails this requirement.

NOTE: The github workflow currently warns about `Unexpected input(s)
'yaml-file-dir'. This is happening because of a bug that doesn't affect
the output. See:
lyubick/action-YAML-schema-validator#8

.github/workflows/validate-yaml-schema.yml

@@ -0,0 +1,22 @@
+on:
+  push:
+    branches:
+      - main
+
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v2
+
+      - name: Check schema
+        uses: lyubick/action-YAML-schema-validator@v2
+        with:
+          json-schema-file: schema.json
+          yaml-file-dir: accounts.yaml
+          recursive: false

schema.json

@@ -0,0 +1,32 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "type": "array",
+  "items": {
+    "type": "object",
+    "properties": {
+      "name": {
+        "type": "string"
+      },
+      "source": {
+        "anyOf": [
+          { "type": "string" },
+          {
+            "type": "array",
+            "items": { "type": "string" },
+            "uniqueItems": true
+          }
+        ]
+      },
+      "accounts": {
+        "type": "array",
+        "items": { "type": "string", "minLength": 12, "maxLength": 12 },
+        "uniqueItems": true
+      },
+      "type": {
+        "enum": ["aws"]
+      }
+    },
+    "required": ["name", "accounts"],
+    "additionalProperties": false
+  }
+}