Add a submodule to provide predefined, purchaseable Giftcard products

Problem/Motivation

💬 Unable to create purchasable gift card Active raised the issue, that the documentation on how to provide purchasable giftcard products is very basic and only targeting developers.

So that may still be a good API-way, but for out-of-the-box experience we should have a submodule that can be installed by non-developers to provide such a "Gift card" product type ootb. If someone doesn't want that and instead create his own implementation, simply don't install the submodule and solve it on your own.

💬 Unable to create purchasable gift card Active should be kept open to improve the documentation on the API part!

Setting priority to major as I think this is a key feature.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes