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/webpack.config.js

@@ -0,0 +1,36 @@
+const path = require('path');
+const nodeExternals = require('webpack-node-externals');
+const TsconfigPathsPlugin = require('tsconfig-paths-webpack-plugin');
+const { NODE_ENV = 'production' } = process.env;
+
+const allowList = [/^netlify-cms-lib-util/];
+
+module.exports = {
+  entry: path.join('src', 'index.ts'),
+  mode: NODE_ENV,
+  target: 'node',
+  devtool: 'source-map',
+  output: {
+    path: path.resolve(__dirname, 'dist'),
+    filename: 'index.js',
+  },
+  resolve: {
+    plugins: [new TsconfigPathsPlugin()],
+    extensions: ['.ts', '.js'],
+  },
+  module: {
+    rules: [
+      {
+        test: /\.ts$/,
+        use: ['ts-loader'],
+      },
+    ],
+  },
+  externals: [
+    nodeExternals({ whitelist: allowList }),
+    nodeExternals({
+      whitelist: allowList,
+      modulesDir: path.resolve(__dirname, path.join('..', '..', 'node_modules')),
+    }),
+  ],
+};