Add support for ezmatrix fields

[bdunogier]

Once ezmatrix has been implemented into eZ Platform, add GraphQL support for it.

A matrix columns are configured at field definition level, meaning that they can be exposed in the schema. With the matrix example from the Legacy doc:

{
  content {
    product {
      components {
        name
        partNumber
        location
      }
    }
  }
}