Add Vuex action to fetch dependencies via GraphQL

Context

As a first step to migrate the frontend for the group-level dependency list, we decided to fetch data from GraphQL.

This can be done on a Vuex-action level (here), which allows making the change with a small potential impact and also applying a feature flag nicely.

graph TD
    B{Feature Flag enabled?}
    B -->|Yes| C[GraphQL Action]
    B -->|No| D[Existing REST Action]
    
    C --> E[GraphQL Query]
    E --> F[Process GraphQL Response]
    F --> G[Update Store]
    
    D --> H[REST API Call]
    H --> I[Process REST Response]
    I --> G
    
    G --> J[Update UI]
    
    subgraph "New Implementation"
        C
        E
        F
    end
    
    subgraph "Existing Implementation"
        D
        H
        I
    end

Implementation Plan

• Add GraphQL query to fetch group-level dependencies
• Add new Vuex action to https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/assets/javascripts/dependencies/store/modules/list/actions.js that uses apollo-client and the above-mentioned action to fetch the data
• Add a feature flag
• If the feature flag is enabled: Use the new action instead of the current one (fetchDependencies)