Scalable Bulk User Creation & Keypair Retrieval Mutation

## Motivation

[Why is this Epic important? Clearly describe the context and reasons for creating this Epic.]

- We need to improve the API to handle cases where there are a very large number of users. In particular, functionalities like bulk user creation in the Control Panel face scaling issues. Currently, user creation sends a separate request for each individual user, and there is no suitable method to retrieve the keypair of the created users, leading to the need to query all keypairs. We must enhance this to ensure that scaling is not an issue.

## Objective

[What is the main goal of this Epic? Describe the desired outcome]

1. When creating a user, the mutation should return the keypair generated during the user creation process.
1. Multiple user creations should be able to be sent in a single mutation.
1. The newly added mutation must be based on UserNode. (There are currently no mutations using UserNode)

JIRA Issue: BA-2237