Fetch clients if not defined in client grants YAML handler

[willvedd]

🔧 Changes

While troubleshooting #855, I noticed that for YAML exports, an exported client grant wouldn't have the client_id swapped with its correlating client's name if the clients didn't exist in the assets object. This should be fairly rare but still technically possible in cases where you're excluding clients from the management purview.

🔬 Testing

Added two new test cases to round-out all possibilities:

1. Clients exist on assets object already, client name replace for ID
2. Clients do not exist on assets, clients fetched but ID not replaced for name because no correlating client exists
3. Clients do not exist on assets, clients fetched and ID replaced for name because correlating client found

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)