Add root users GraphQL search and associated edges

What does this MR do?

Adds the following things to the GitLab GraphQL API

• users as a root query. It can be used to retrieve a single user by username or every user on that instance when authenticated as an administrator. e.g.

{
  users(username: "user12299") {
    id
  }
}

• memberships to the UserType class, to traverse the graph to all members belonging to a user. This is a polymorphic association and so can belong to a number of types. It allows for queries such as:

{
  users(username: "user12299") {
    nodes {
      id
      memberships {
        nodes {
          sourceType
        }
      }
    }
  }
}

• A MemberType to the GraphQL schema. This represents the polymorphic association between a User and the types that they can be a "member" of, such as Project or Group.

• Several simple attributes on MemberType.

• A Source attribute on MemberType which traverses to the source value. This should be a GraphQL Union type to enable more effective querying of the API.

• Ensure that authorization is working as expected, as this potentially exposes a lot of data about an instance's users! Consider a security review.

• An example GraphQL query to enable output as defined in the parent issue.

Proposed query

{
  users(username: "user12299") {
    nodes {
      id
      memberships {
        nodes {
          ...membership
          ...source
          ...creator
        }
      }
    }
  }
}

fragment membership on Member {
  createdAt
  updatedAt
  accessLevel
  sourceType
  expiresAt
}

fragment creator on Member {
  createdBy {
    id
  }
}

fragment source on Member {
  source {
    ... on Group {
      id
    }
    ... on Project {
      id
    }
  }
}

• Added documentation to the GraphQL API docs.

Screenshots

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.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #215658 (closed)