Implement custom drop downs on clicking nb-action element

### Issue type

**I'm submitting a ...** (check one with )

* [x] feature request
* [ ] bug report

### Issue description

**Current behavior:**
Not able to use custom drop downs with NbActions

**Expected behavior:**
Should be able to use user defined dropdowns when clicked on any action from nb-actions element

**Related code:**

```

```

On clicking this i want to show a similar dropdown as given below:


### Other information:

**Angular, Nebular**
```
"@angular": "~10.2.0",
"@nebula": "^6.2.1",
```