Feat: indexed props

[hbarcelos]

This PR includes item props into the subgraph. Since there's defined no schema for items in a list, parameters are defined roughly as if they were in the 4th Normal Form.

A query for items with props will look like:

{
  items {
    id
    props {
      type
      label
      description
      isIdentifier
      value
    }
  }
}

A couple of things to consider:

• There is no support for union types, so value is always a string. It's up to the UI to figure out the right type based on the type prop.
• There is no support for value objects in The Graph, so every prop is an entity whose ID is <gtcr_addr>@<item_id>@<prop_label>. I'm not sure how this impacts performance.

[hbarcelos]

Also notice that this feature is dependent on PR #177 on the GTCR UI repo, since the file must be available at the managed The Graph IPFS node.