Search issues in graphQl by milestone and assignees

What does this MR do?

It adds support to graphQL API for filtering issues by milestones (milestone_title) and assignees (asisgne_username, assignee_id)

GraohQL query example

project(fullPath: "flightjs/flight") {
  name
  issues(state: opened, milestoneTitle: "Sprint", assigneeId: "none") {
    edges {
      node {
        iid
        title
      }
    }
  }
}

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

This is backend part of #197227 (closed)