Decouple customizable dashboards from product analytics

What does this MR do and why?

Analytics dashboards can have the following dashboards

• Built-in product analytics dashboards (behaviour and audience)
• Value stream dashboards
• Custom dashboards defined by user

Currently, all of these are controlled with product analytics feature flag. However, only built-in product analytics dashboards (behavior and audience) should depend on the feature flag value.

In addition, if the feature flag is enabled but onboarding is incomplete (no events in clickhouse), dashboards graphql response should not include product analytics dashboards.

This MR makes the above-mentioned decoupling.

There will be a follow up MR for the frontend changes.

Screenshots or screen recordings

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List query

FF enabled	FF disabled

Fetch with slug value_stream_dashboard (FF flag enabled or disabled, both cases)

How to set up and validate locally

1. Setup product analytics
2. Go to GraphiQL explorer

query getAllProductAnalyticsDashboards($projectPath: ID!) {
  project(fullPath: $projectPath) {
    id
    customizableDashboards {
      nodes {
        title
      }
    }
  }
}

1. It should return value stream dashboard
2. Enable feature flag

Feature.enable(:product_analytics_dashboards, Project.find(project_id))

1. Do the same graphQL query
2. You should receive 'behavior', 'audience', and 'value stream analytics'.

MR acceptance checklist

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

Related to #424440 (closed)