[2.x] Allow file uploads through file fields

[Jade-GG]

ref: BORDEX-1459 & BORDEX-1448

This seemed relatively simple at first, but then ended up being a somewhat complicated problem. When you request a quote of a product that has a file field, it won't add the file to the quote data.

I've had to consider 2 different possibilities:

• You can request a quote from your cart. This means the file is uploaded to the Magento server, and thus can be easily retrieved from there. (Note that this currently requires Magento to be on the same container/server as Rapidez)
• You can request a quote directly from a product page. This means the file isn't yet uploaded anywhere, and as such we save it to an IndexedDB keyval store.

After this, the file gets uploaded to Statamic by using the container's flysystem functionality, making sure to restrict file size and file extensions.

[Jade-GG]

Note that I will add a 5.x version once this one has been merged. I started on this version because that's the version that the customer that requested this feature is on.