[VSA] Add GraphQL query for Value Streams

Task

Implement a project and group level GraphQL query which returns the available value streams.

Structure

group(fullPath: "gitlab-org") { // or project
  valueStreamAnalytics {
    valueStreams(id: '') {
      id // global id
      name
      createdAt
      updatedAt
    }
  }
}

Notes:

• Returns an array of value streams for the given group or project
• The endpoint is not paginated. We expect low number of rows and we're planning to add limits: #409609 (closed)
• id argument is optional for loading one particular VS.
• The value streams are sorted by name

FOSS VS EE

VSA is also available on the project-level (FOSS), however there are no persisted value streams. The endpoint should return an in-memory value stream with id=default. This is the same behavior as today with the private REST API.

Rules:

• FOSS: valueStreamAnalytics is available on project only. Returns one in-memory stage. The id filter (id=default) would should work.
• EE: valueStreamAnalytics is available on both project and group. Make sure to the usual license checks: Gitlab::Analytics::CycleAnalytics.licensed? and Gitlab::Analytics::CycleAnalytics.allowed?