Expose issue count flow metric via GraphQL

What does this MR do and why?

This MR sets up the resolver structure for exposing value stream flow metrics via GraphQl on both project and group levels. Currently, VSA has private APIs which are consumed by the front end only.

This MR exposes the issue count metric. The same count could be requested via the standard issues { count } query however, we decided to separate the VSA API because the long-term plan is to provide aggregated metrics with different visibility rules.

I've marked the API as alpha for now, until we add all metrics.

Screenshots or screen recordings

How to set up and validate locally

1. Ensure you're on ultimate plan (this is needed for the group-level query)

query groupLevelQuery {
  group(fullPath: "flightjs") {
    flowMetrics {
      issueCount(projectIds: ["asdf"], from: "2022-01-01", to: "2022-01-02") {
        value
        title
        identifier
        links
      }
    }
  }
}

query projectLevelQuery {
  project(fullPath: "gitlab-org/gitlab-test") {
    flowMetrics {
      issueCount(from: "2022-01-01", to: "2022-03-01") {
        value
        title
        identifier
        links
      }
    }
  }
}

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 #384559 (closed)