Use fingerprinted file name for microfrontend entrypoint bundle to enable caching it

[xkons]

This PR adds caching for the micro frontend bundles on top of https://github.com/fboeller/microfrontends-with-angular/pull/1

To make our our micro frontends cacheable, we adjust their build step to generate the main.js entrypoint file with a content based hash in the filename. This can easily be done using the Angular CLI. Then we just need to store the information of this filename in a file on the static storage (e.g. netlify) of the micro frontend so that we can load it. This new frontend-meta.json just weighs 1kB and will not be cached, so that it always contains the latest hashed entrypoint file name.

When loading a micro frontend, we now first load the frontend-meta.json and then the fingerprinted main.<hash>.js file. The latter will be cached by the browser. So until we deploy a new version of a micro frontend, the browser of our users will not have to download the roughly 500kB to open a page with that micro frontend anymore, but it is directly loaded from the users cache stored on their computer.

Important to know: The frontend-meta.json should never be cached by your clients to ensure that they always get the latest entrypoint bundle of your microfrontend.

This PR enables us to add a caching header to the response that fetches the entrypoint bundle of a microfrontend like.

Since the file name of the entrypoint bundle contains a md5 hash based on the content of the file, we know that if a bundle with the same name has been fetched by the browser, it is unchanged.

E.g. caching for one year: Cache-Control: max-age=31536000

The main.js file name we used before did not allow for caching, while still ensuring that all customers always get the latest micro frontend code.