[prototype] natural content filtering

[bdunogier]

The collection field for a type (articles, blogPosts) will expose an argument
for each searchable field:

{
  media {
    images(name: "~norway") {
      _name
    }
  }
  content {
    products(price: "> 20", gluten: false) {
      _name
    }
  }
}

Supported operators (as first characters of the string): ~ (like, with automatic wildcards), <, >, <=, >=).

TODO

• Add syntax for not (!~norway)
• Accept array of values arguments where at least one value has an operator as a OR.
• Handle date fields (publicationDate: "< 2018/11/10")
• Refactor fields filters processing, and ideally unit test it.
• Figure out what's "searchable" when the LSE is used. It is actually detected when converting the criteria using the LegacyConverter's getIndexColumn() method, that returns false for these fields. It may have to be taken into account at schema generation's time, but the schema will then depend on the environment / search engine.
• Reintroduce non-field filters (fulltext, parent location...), probably prefixed with _
• Add fields to sortBy as well

[bdunogier]

Moved to ezsystems/ezplatform-graphql#5.