networkd: Add support to configure destination address for bridge FDB

Closes #5145.

Example conf:
```
[Match]
Name=vxlan1309

[BridgeFDB]
MACAddress=00:00:00:00:00:00
Destination=10.0.0.2

[BridgeFDB]
MACAddress=00:00:00:00:00:00
Destination=10.0.0.4

[BridgeFDB]
MACAddress=00:00:00:00:00:00
Destination=10.0.0.5
```