feature: Custom Extensions

[tusharmath]

This feature should allows developers to write custom Rust functions to extend Tailcall's ability.
Here is how the user experience going to look like:

Example

Let's say we want to implement langugage translation in Tailcall.

The extension is loaded using the link operator

schema
  # Dynamically Link an Extension
  @link(src: "./translate", type: Extension)
  @upstream(baseURL: "https://jsonplaceholder.typicode.com") {
  query: Query
}

type Query {
  users: [User] @http(path: "/users")
  user(id: String): [User]
    @http(path: "/users")
    # User can use the @extension directive
    @extension(name: "some_other_extension", params: ["{{.args.id}}"])
}

type User {
  id: ID
  name: String
    @extension(
      name: "translate"
      params: ["userName", "{{.value.catchPhrase}}"]
    )
  username: String
  company: Company
}

type Company {
  name: String
  catchPhrase: String
    @extension(
      name: "translate"
      params: ["catchPharse", "{{.value.catchPhrase}}"]
    )
  bs: String
}

Directive Definition
Should be added automatically to the .tailcallrc.graphql

directive @extension(name: String!, params: [String!]!) on FIELD

Rust Code

The extension needs to implement the following trait:

trait Extension {
    // Initializes the extension
    async fn init(&self);

    // Called before the IR is going to be executed
    fn prepare<Json: JsonLikeOwned>(&self, ir: IR, params: &[Json]) -> IR;

    // Called after a value is produced
    fn process<Json: JsonLikeOwned>(&self, params: &[Json], value: Json) -> Json;
}

The trait is exposed via tailcall::extensions module that published as a crate. The extension that implements the trait Extension should loaded dynamically into Tailcall.

Technical Requirements

• Add tests and documentation wherever possible
• Use lib loading should be async and failures should be nicely displayed to the user

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