wasm: Add request body in the form of Bytes (#696)

* Add body bytes

* Add example and header creation code

examples/wasm_header/webpack.config.js

@@ -0,0 +1,25 @@
+const path = require('path');
+const HtmlWebpackPlugin = require('html-webpack-plugin');
+const webpack = require('webpack');
+const WasmPackPlugin = require("@wasm-tool/wasm-pack-plugin");
+
+module.exports = {
+    entry: './index.js',
+    output: {
+        path: path.resolve(__dirname, 'dist'),
+        filename: 'index.js',
+    },
+    plugins: [
+        new HtmlWebpackPlugin(),
+        new WasmPackPlugin({
+            crateDirectory: path.resolve(__dirname, ".")
+        }),
+        // Have this example work in Edge which doesn't ship `TextEncoder` or
+        // `TextDecoder` at this time.
+        new webpack.ProvidePlugin({
+            TextDecoder: ['text-encoding', 'TextDecoder'],
+            TextEncoder: ['text-encoding', 'TextEncoder']
+        })
+    ],
+    mode: 'development'
+};