Allow setting of weight on work item create

What does this MR do and why?

Adds GraphQL argument to allow weight widget input on create

MR acceptance checklist

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How to set up and validate locally

Sample GraphQL query:

mutation {
  workItemCreate(
    input: {
      namespacePath: "gitlab-org/gitlab-test",
      title: "Test work item",
      workItemTypeId: "gid://gitlab/WorkItems::Type/6",
      weightWidget: {
        weight: 5
      }
    }
  ) {
    clientMutationId
    workItem {
      id
      title
      widgets {
        type
        ... on WorkItemWidgetWeight {
          weight
        }
      }
    }
  }
}

Related to #498071 (closed)