Delete and Update functionality for tree grid

### Issue type

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

* [ ] bug report
* [X] feature request

### Issue description

**Current behavior:**
You have to make changes to the data which was used by the `NbTreeGridDataSourceBuilder` and then re-instantiate the `NbTreeGridDataSource` of the TreeGrid

**Expected behavior:**
Add functions to `NbTreeGridDataSource` to update or delete entries

**Steps to reproduce:**
Something like this would be nice
```ts
NbTreeGridDataSource.update(target: T, updated: T)
NbTreeGridDataSource.delete(target: T, options?: {deep?: boolean})
```