Add support for custom hook points for extensions

[timsuchanek]

Use-case: After fetching a big schema with get-schema, it would be awesome to add app specific annotations to that generated SDL. An extension could hook in and manipulate the downloaded SDL of the graphql get-schema command.

[ardatan]

Currently we have new structure for GraphQL CLI and in the new alpha version 4.0.0-alpha.0 we suggest to use GraphQL Code Generator plugin of GraphQL CLI with Codegen's schema-ast plugin. GraphQL Codegen has lifecycle hooks feature.
Checkout new instructions and we will be happy to hear your feedback. Feel free to contact us using our Discord channel;
https://discord.gg/xud7bH