closing all open NbDialogRef

### Issue type

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

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
NbDialogRefs stays open when routing changed.

**Expected behavior:**
having option to get all NbDialogRefs so I can close all when routing changed, without checking if routing changed for every NbDialogRef in project.

**Steps to reproduce:**
open NbDialogRef in angular from angular component, then change routing that makes the router-outlet to switch component, but the dialog stays open even its not relevant anymore.

**Related code:**
this.dialogSrv.open(UsageReportDialogComponent)
this.router.navigateByUrl(`home/devices');
```
insert short code snippets here
```

### Other information:

**npm, node, OS, Browser**
```

Node: v14.17.3
npm: 6.14.13
OS: 10
Browser: Chrome
```

**Angular, Nebular**
```
Angular: 11.2.13
Nebular: 8.0.0
```