Make improvements for graphql plugin

[Dmitriy Lazarev]

During integration it to HP project I noticed that our graphql plugin has some flaws

• It handles graphql schema directives only at runtime, but we want it to be able generate correct types from final schema and also provide final schema for clients
• Under the hood it uses catalog client for a data loader, but we have a better and more performant batch loader
• It doesn't allow to provide custom loader either. It's needed when you have multiple sources which aren't ingested into backstage catalog