title: @Id field annotation for storing POJO in Firebase Firestore?
body: <p>We have a complex JSON having multiple sub-objects inside main object.
  For example:</p>
  
  <pre><code>dept: {
     name: Dept1,
     emps: [
       {
          empCode: 001,
          empName: Emp001
       }
     ]
  }
  </code></pre>
  
  <p>If we use <code>db.collection("dept").add(deptPojo)</code> then it auto creates "emps" subcollection and auto assigns document id for each emp object.
  But we want empCode to be assigned as document id for each emp document.
  Is this achievable? How? Is there any @Id annotation in firebase firestore api?</p>