IPAM | VRF add VNI field

### NetBox version

v4.4.8

### Feature type

Data model extension

### Proposed functionality

VRFs already support assigning route-targets and route-distinguishers.

### Use case

In EVPN VXLAN deployments a VRF also receives a VNI (VXLAN Network Identifier), add a VNI field with 24bit support

### Database changes

_No response_

### External dependencies

_No response_