[typescript-generic-sdk] Return Type needs to be Promise<AsyncIterable<R>>

### Which packages are impacted by your issue?

@graphql-codegen/typescript-generic-sdk

### Describe the bug

I was trying to implement a requester in nodejs with apolloClient in typescript (see my gist).

With a subscription being returned as an `AsyncIterable`, there is no way of knowing, when the subscription has been established/is listening.
Making the return type `Promise>` would provide the proper interface to represent the availability of the connection.
Awaiting for the first `[Symbol.asyncIterator]().next()` isn't the same, as a subscription might omit onConnect data afaik.
Also it's kind of unhandy syntax compared to just using an additional await.

With that in place one should be able to run a mutation after the subscription is established, to not miss the message.

https://gist.github.com/engelant/3f976774b5d1c6d9848e7ad346ef913a

### Your Example Website or App

https://github.com/dotansimha/graphql-code-generator-community/tree/main

### Steps to Reproduce the Bug or Issue

1. Generate sdk code with typescript-generic-sdk plugin
2. The return type of a subscription is innsufficient to represent the connection state

### Expected behavior

As a user I would like to be able to easily ensure my subscription is established before executing further code.

### Screenshots or Videos

_No response_

### Platform

https://github.com/dotansimha/graphql-code-generator-community/tree/main

### Codegen Config File

_No response_

### Additional context

_No response_

Start in the @graphql-codegen/typescript-generic-sdk package by locating where subscription return types are defined and generated. Use the linked gist to understand the requester scenario, then verify that generated subscriptions expose Promise> and let callers wait for establishment before running subsequent code.

Written by the indexing model from the issue text.