Add initial API for Duo Pro bulk assign

What does this MR do and why?

This MR:

• Add API for Duo Pro bulk assign
• Add the GQL API and mutation

Mutation Example:

mutation {
  userAddOnAssignmentBulkCreate(
    input: {userIds: ["gid://gitlab/User/1"], addOnPurchaseId: "gid://gitlab/GitlabSubscriptions::AddOnPurchase/6"}
  ) {
    addOnPurchase {
      id
    }
    users {
      edges {
        node {
          id
          name
          username
        }
      }
    }
    errors
  }
}

Response types:

NOT_ENOUGH_SEATS

INVAILD_USER_ID_PRESENT

Success

Related to #415586