Add Helix implementation using @apollo/subgraph (#235)

Introduces Helix to the testing repo leveraging `@apollo/subgraph`'s
`buildSubgraphSchema` function, giving full Fed 1 / Fed 2 compatibility
(excluding federated tracing).

Author: hwillson

implementations/helix/Dockerfile

@@ -0,0 +1,9 @@
+FROM node:14-alpine
+WORKDIR /web
+COPY package.json package-lock.json products.graphql tsconfig.json ./
+COPY src ./src
+RUN npm install
+RUN npm run build
+EXPOSE 4001
+USER node
+CMD node dist/index.js

implementations/helix/docker-compose.yaml

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+services:
+  products:
+    # must be relative to the root of the project
+    build: implementations/helix
+    ports:
+      - 4001:4001

implementations/helix/metadata.yaml

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+fullName: GraphQL Helix
+language: JavaScript / TypeScript
+documentation: https://graphql-helix.vercel.app