OSPFv3: add support for NSSA area-type
Supported in FRR 8.1
FRR:
```
LR1.wue3(config-ospf6)# area 0.0.0.0
export-list Set the filter for networks announced to other areas
filter-list Filter networks between OSPF6 areas
import-list Set the filter for networks from other areas announced to the specified one
nssa Configure OSPF6 area as nssa
range Configured address range
stub Configure OSPF6 area as stub
LR1.wue3(config-ospf6)# area 0.0.0.0 nssa
<cr>
default-information-originate Originate Type 7 default into NSSA area
no-summary Do not inject inter-area routes into area
```