Add GraphQL Support for Handling Virtual Folder Invitations

## Feature Request

### Summary

We would like to request the addition of GraphQL (GQL) support for accepting or rejecting virtual folder (vfolder) invitations. Currently, this functionality can be somewhat managed through the REST API, but having dedicated GQL mutations and queries would significantly enhance usability and integration within the [http://Backend.AI](http://Backend.AI) platform.

### Detailed Explanation

The specific functionalities we are looking for include:

1. **Query to List Pending vFolder Invitations**: A query that returns a list of vfolder invitations currently awaiting response, along with a total count of such invitations. The output should at least include the following fields, but can be modeled after the existing `/invitations/list` REST API response or the `vfolderNode` structure:
- `inviter`: The user who sent the invitation
- `invitee`: The user to whom the invitation was sent
- `perm`: Permissions associated with the invitation
1. **Mutations to Accept or Delete Invitations**: We need mutations to:
- Accept a vfolder invitation
- Delete or reject a vfolder invitation

These additions would streamline the process of managing vfolder collaborations and permissions, aligning with the user-friendly and flexible nature of [http://Backend.AI](http://Backend.AI) 's platform.

### Use Case

This feature is crucial for users looking to manage their virtual folder collaborations more efficiently within the [http://Backend.AI](http://Backend.AI) Web UI, without having to resort to the REST API for these specific actions. It will facilitate a smoother workflow for accepting or declining folder shares, especially in environments where collaboration and resource sharing are frequent.

JIRA Issue: BA-1069