Process and display all product analytics dashboards

What does this MR do and why?

Adds a few pieces to the puzzle to deserialize dashboard definitions in to usable Ruby objects and make them available via our GraphQL API.

📺 Quick demo on unfiltered: https://youtu.be/FKcYiT4h3m8

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:cube_api_proxy)

2. Make sure the following application settings are set. (Their values aren't important for this endpoint.)

• product_analytics_enabled == true
• cube_api_base_url
• cube_api_key

1. In any repository, create a dashboard definition by creating a file .gitlab/product_analytics/dashboards/north_star.yaml with this content:

---
version: '1'
title: Dashboard Example 1
description: North Star Metrics across all departments for the last 3 quarters.
widgets:
  - title: Overall Conversion Rate
    gridAttributes:
      yPos: 4
      xPos: 1
      width: 12
      height: 2

1. Make an API call to that project using GraphQL authenticated using a developer or higher on that project.

query {
  project(fullPath: "flightjs/project_twenty") {
    name
    productAnalyticsDashboards {
      nodes {
        title
        description
        widgets {
          nodes {
            title
            gridAttributes
          }
        }
      }
    }
  }
}

1. Assert that the dashboard title and description is returned.
2. Attempt to also find a single dashboard:

query {
  project(fullPath: "flightjs/project_twenty") {
    name
    productAnalyticsDashboards(slug: "dashboard_slug") {
      nodes {
        title
        description
        widgets {
          nodes {
            title
            gridAttributes
          }
        }
      }
    }
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #379669 (closed)