[BC break] feature: Coreshop store support

[dkarlovi]

This PR changes the configuration for the bridge to better align with Coreshop names for concepts:

• removes store_aware config done in #50, now it's store-aware by default (BC break)
• stores are renamed to sites (BC break)
• stores added inside newly formed sites, pointing to Coreshop stores (currently via name)
• a new interface, StoreAwareDocumentMapperInterface, allowing the mapper to declare it want the current store when normalizing
• DocumentProductMapper migrated to this interface, it accepts the store and fetches the store-specific price

Config looks like this:

core_shop2_vue_storefront:
    sites:
        example.nl:
            stores:
                - example.nl # store name, must be the same as Coreshop store name
            languages:
                - nl
        example.co.uk:
            stores:
                - example.co.uk
            languages:
                - en
        example.com:
            stores:
                - example.com EUR
                - example.com USD
            languages:
                - en
        example.de:
            stores:
                - example.de
            languages:
                - de
        example.es:
            stores:
                - example.es
            languages:
                - es
        example.fr:
            stores:
                - example.fr
            languages:
                - fr

[dkarlovi]

@siwamhader could you test this out to see if it makes sense for you?

[ghost]

@dkarlovi looks good to me.