Support for metaTitle, metaDescription and keywords for landing page

[mkucmus]

PROBLEM: Currently, there is no way to fetch meta-related info for every landing page, using store-api within page resolver.

BACKGROUND: Probably it’s because there is no entity object (for langing page) attached to the response, as it’s done for other types like product and category (the product and category objects are present in the response and meta related info can be extracted later on).

The mentioned fields are available when the built-in endpoint is used /store-api/landing-page/{landing-page-id}

related: https://github.com/vuestorefront/shopware-pwa/issues/1863