Enable dependabot on the Rust projects

Add a dependabot.yml file to trigger daily dependabot updates on the
four Rust projects in the codebase:

- `node_types`
- `generator`
- `extractor`
- `autobuilder`

Author: hmac

.github/dependabot.yml

@@ -0,0 +1,18 @@
+version: 2
+updates:
+  - package-ecosystem: "cargo"
+    directory: "/node-types"
+    schedule:
+      interval: "daily"
+  - package-ecosystem: "cargo"
+    directory: "/generator"
+    schedule:
+      interval: "daily"
+  - package-ecosystem: "cargo"
+    directory: "/extractor"
+    schedule:
+      interval: "daily"
+  - package-ecosystem: "cargo"
+    directory: "/autobuilder"
+    schedule:
+      interval: "daily"