add webpack plugin to manage the externals configuration

[karthik2804]

Once this is merged, the webpack config on apps can become something like the following where the plugin takes care of injecting the externals.

const path = require('path');
const SpinSDKPlugin = require("@fermyon/spin-sdk/plugin/webpack")

module.exports = {
    ...
    plugins: [
        new SpinSDKPlugin()
    ],
    ...
};