add conditional app block

[veken1199]

We are introducing a sample app block that uses available_if property. This block is conditionally enabled on storefronts only when the referenced app-owned metafield has a value of true.

A partner can create an app-owned metafield using metafieldSet mutation of the 2022-04 Admin API or above.

Example mutation:

mutation metafieldsSet($metafields: [MetafieldsSetInput!]!) {
  metafieldsSet(metafields: $metafields) {
    metafields {
      # Metafield fields
    }
    userErrors {
      field
      message
    }
  }
}

Mutation Variables:

{
  "metafields": {
    "key": "feature1",
    "namespace": "sample_app",
    "ownerId": "<App installation ID>",
    "type": "boolean",
    "value": "true"
  }
}

In order to get app installation ID, use the following Admin API GraphQL query:

{
  appInstallation {
        id
  }
}

[veken1199]

/shipit