title: How to get all sortable columns from MatSort
body: <p>I have data table and I want to apply the sorting process from the back-end side, so the end user can apply sorting on multiple columns, what I need to do is get all <code>columns</code> with their sorting values <code>asc/desc</code> on each <code>sortChange</code> event. </p>
  
  <p>lets say the columns are:
   <code>name, gender, date_of_birth.</code>
  results should be:</p>
  
  <pre><code>[
  {'name', 'asc'},
  {'gender', 'desc'},
  {'date_of_birth', 'desc'},
  ]
  </code></pre>
  
  <p>ts code:</p>
  
  <pre><code>ngOnInit() {
   this.getData([])
   this.sort.sortChange.subscribe(
              () =&gt; {
              // access all the sortable columns
              let sortable_columns = -------------- // what should I write here?
              // call the http service
              this.getData(sortable_columns)
              });
  }
  </code></pre>
  
  <p>Now I think that the general alogrithm is fine, but I can't find the right syntax to access the sortable columns, any Idea on how to do this? 
  Thanks.</p>