Expose last_synced of DataSources in the Rest api

### NetBox version

v4.0.5

### Feature type

Change to existing functionality

### Proposed functionality

Extend the DataSource representation in the api with the `last_synced` field.

### Use case

I would like to monitor if all the are up-to-date. Beside the state the age of the Datasource 

### Database changes

No

### External dependencies

None