title: Display Pre-ApplyWarning when changing specific properties
body: <p>Is it possible to display pre-apply warnings to the user using the Terraform plugin SDK?</p>
  <p>For example: Changing the RAM size of a VM resource requires the VM to be shutdown. Therefore I'd like to display a warning to the user that the VM will be restarted during the update. This should happen <em>before</em> asking for approval and not after applying the changes.</p>
  <p>I tried checking for changes in the resource config in the <code>ReadContextFunc</code>, but it didn't do do anything in the :</p>
  <pre class="lang-golang prettyprint-override"><code>func resourceRead(ctx context.Context, d *schema.ResourceData, m interface{}) diag.Diagnostics {
      // ...
  
      if d.hasChanges(&quot;foo&quot;) {
          diags = append(diags, diag.Diagnostics{
              Severity: diag.Warning,
              Summary: &quot;Changing foo is a crime&quot;
          }
      }
  
      // ...
  }
  </code></pre>