Refactor `FieldValueBuilder` like it was done in graphql-page

graphql-page has a [mechanism similar to FieldValueBuilders](https://github.com/bdunogier/ezplatform-graphql-page/blob/schema_refactoring/src/GraphQL/Schema/BlockAttributeBuilder.php). It comes with a "Configurable" flavor that handles each block type with its own service, and should be easier to maintain. Apply the same to this package.