Add vue-storefront-api output cache

[pkarw]

What is the motivation for adding / enhancing this feature?

Vue Storefront does support the tagged output caching. Would be great to add the same feature to the API, to limit the number of ElasticSearch queries - and having the tags to easily invalidate the cache.

Reference PR: #1731

What are the acceptance criteria

• the vue-storefront-api catalog endpoint supports the Redis based output cache
• vue-storefront-api is setting the proper tags of [entityType(0)+id](https://github.com/DivanteLtd/vue-storefront-api/blob/d7b6fe516eeb214615f54726fb382e72ff2cc34b/src/api/catalog.js#L1050 for example: P124 for the product with id=124 along with each output request
• there is additional vue-storefront-api endpoint for clearing out the cache - it could be a copy of vue-storefront/invalidate
• there is an additional script in the API CLI to clear out the cache - it should be just a copy of vue-storefront cache.js
• vue-storefront invalidateCache is calling the vue-storerfront-api invalidate cache endpoint in here

Can you complete this feature request by yourself?

• YES
• NO

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

Pick one option.

• 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.
• (Pick this option only if you're sure) This is an important improvement request for current Release Candidate version on https://next.storefrontcloud.io and should be placed in next RC version. In this case Developer should create branch from release branch and create Pull Request 3. Stabilisation fix back to release.
• (Pick this option only if you're sure) This is a critical improvement request for current Stable version on https://demo.storefrontcloud.io and should be placed in next stable version. In this case Developer should create branch from hotfix or master branch and create Pull Request 4. Hotfix back to hotfix.

Additional information