Add compatibility with Mercure protocol

[mattheo-geoffray]

What is the motivation for adding / enhancing this feature?

Mercure repository: https://github.com/dunglas/mercure

The Mercure protocol allows to push data updates to web browsers and other HTTP clients.

It can publish real-time updates of resources served through web APIs, to reactive web and mobile apps.

It is built on top of HTTP so it supported by native browsers

One example for VSF is the one explained on the Mercure repository:

- a webapp retrieves the availability status of a product from a REST API and displays it: only one is still available
- 3 minutes later, the last product is bought by another customer
- the webapp's view instantly shows that this product isn't available anymore

What are the acceptance criteria

/

Can you complete this feature request by yourself?

• [ ] YES
• [ ] NO

Which Release Cycle state this refers to? Info for developer.

• [x] This is a normal feature request. This should be available on https://test.storefrontcloud.io and then after tests this can be added to next Vue Storefront version. In this case Developer should create branch from develop branch and create Pull Request 2. Feature / Improvement back to develop.

Additional information

For now it is compatible only with React but there is a draft for Vue.js: https://github.com/api-platform/client-generator/issues/100

Here is the PR for Vue.js compatibility: https://github.com/api-platform/client-generator/pull/114