Add ability to uniquelly identify a work item by path and iid pair

What does this MR do and why?

Add ability to uniquelly identify a work item by path and iid pair

Add the ability to identify uniquelly a work item by namespace full path and work item iid.

This can be done by using a root level work_item query or using the group, project or namespace level work_item field.

MR acceptance checklist

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How to set up and validate locally

query root_level {
  groupLevelByID: workItem(id: "gid://gitlab/WorkItem/350") {
    id
    title
    namespace {
      fullPath
    }
  }
  projectLevelByID: workItem(id: "gid://gitlab/WorkItem/89") {
    id
    title
    namespace {
      fullPath
    }
  }
  groupLevelByPathAndIID: workItem(
    namespacePathWithIid: {fullPath: "toolbox", iid: 20}
  ) {
    id
    title
    namespace {
      fullPath
    }
  }
  projectLevelByPathAndIID: workItem(
    namespacePathWithIid: {fullPath: "toolbox/gitlab-smoke-tests", iid: 20}
  ) {
    id
    title
    namespace {
      fullPath
    }
  }
}

query group_level {
  group(fullPath: "toolbox") {
    id
    name
    workItem(iid: "20") {
      id
      title
    }
  }
}

query project_level {
  project(fullPath: "toolbox/gitlab-smoke-tests") {
    id
    name
    workItem(iid: "20") {
      id
      title
    }
  }
}

query group_namespace_level {
  namespace(fullPath: "toolbox") {
    id
    name
    workItem(iid: "20") {
      id
      title
    }
  }
}

# this does not work yet because Resolvers::NamespaceResolver does not resolve projects by full path yet
query project_namespace_level {
  namespace(fullPath: "toolbox/gitlab-smoke-tests") {
    id
    name
    workItem(iid: "20") {
      id
      title
    }
  }
}

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