[IJ Plugin] Specific Gradle DSL for endpoints used by the IDE plugin

[BoD]

Currently, the IDE plugin will look at the end-point and headers configured in the introspection block:

    introspection {
      schemaFile.set(file("src/commonMain/graphql/schema.graphqls"))
      endpointUrl.set("https://confetti-app.dev/graphql")
      headers.set(mapOf("conference" to "androidmakers2024"))
    }

and use them for:

1. fetching the introspection (via Tools|Apollo|Download schema)
2. executing the queries (via the 'play' button in the UI)

1 makes sense but 2 is a bit surprising, since it has nothing to do with introspection. Also it would be useful to be able to configure some headers only for execution, and not introspection (i.e. here).

With the GraphQL plugin, the endpoints and headers can be configured with the graphql.config.yml file, but Apollo plugin users are told to not use that file (as the conf is contributed to the GraphQL plugin by the Apollo plugin, using the project's Gradle conf).
(Question: what's the behavior currently when a graphql.config.yml file is present? I think it's ignored, to be confirmed.)

We could have a specific DSL to configure the plugin, syntax TBD, something like:

idePlugin {
  endPoints {
	  endPoint {
	    url.set("...")
	    headers.set(...)
	  }
  }
}