chore: add proxy backend (#3126)

* feat(backends): add proxy backend

* feat: add proxy server initial commit

* fix: move from joi to @hapi/joi

* test: add joi validation tests

* feat: proxy server initial implementations

* test: add tests, fix build

* chore: update yarn.lock

* build: fix develop command

* fix(back-proxy): fix bugs

* test(backend-proxy): add cypress tests

* chore: cleanup

* chore: support node 10

* chore: code cleanup

* chore: run cypress on ubuntu 16.04

* test(e2e): fix proxy backend cypress tests

* chore: don't start proxy server on yarn develop

packages/netlify-cms-proxy-server/package.json

@@ -0,0 +1,54 @@
+{
+  "name": "netlify-cms-proxy-server",
+  "description": "Proxy server to be used with Netlify CMS proxy backend",
+  "version": "1.0.1",
+  "repository": "https://github.com/netlify/netlify-cms/tree/master/packages/netlify-cms-proxy-server",
+  "bugs": "https://github.com/netlify/netlify-cms/issues",
+  "license": "MIT",
+  "main": "dist/index",
+  "keywords": [
+    "netlify",
+    "netlify-cms",
+    "backend"
+  ],
+  "sideEffects": false,
+  "scripts": {
+    "build": "webpack",
+    "prestart": "yarn build",
+    "start": "node dist/index.js",
+    "develop": "nodemon --watch 'src/**/*.ts' --ignore 'src/**/*.spec.ts' --exec 'ts-node' --files src/index.ts",
+    "test": "jest",
+    "test:watch": "yarn test --watch",
+    "test:coverage": "yarn test --coverage"
+  },
+  "dependencies": {
+    "@hapi/joi": "^17.0.2",
+    "cors": "^2.8.5",
+    "dotenv": "^8.2.0",
+    "express": "^4.17.1",
+    "joi": "^14.3.1",
+    "morgan": "^1.9.1",
+    "netlify-cms-lib-util": "^2.3.0",
+    "simple-git": "^1.129.0"
+  },
+  "devDependencies": {
+    "@types/cors": "^2.8.6",
+    "@types/express": "^4.17.2",
+    "@types/hapi__joi": "^16.0.6",
+    "@types/jest": "^24.9.0",
+    "@types/morgan": "^1.7.37",
+    "@types/node": "^13.1.7",
+    "@types/vfile-message": "^2.0.0",
+    "jest": "^24.9.0",
+    "nodemon": "^2.0.2",
+    "ts-jest": "^24.3.0",
+    "ts-loader": "^6.2.1",
+    "ts-node": "^8.6.2",
+    "tsconfig-paths-webpack-plugin": "^3.2.0",
+    "webpack": "^4.41.5",
+    "webpack-node-externals": "^1.7.2"
+  },
+  "engines": {
+    "node": ">=v10"
+  }
+}