route: Introduce `state: ignore` for route

Given a system with ignored interface holding static routes,
When applying desire state retrieved by `nmstatectl show` or
`libnmstate.show()`,
The nmstate will fail with error:

```
    Desired route destination: 2001:db8:e::/64 next-hop-interface: eth1
    next-hop-address: 2001:db8:1::2 table-id: 254 not found after applyVg
```

This is caused by nmstate removed ignored interface during merging
desired state with current.

To fix this issue, this patch introduced `state: ignore` to route entry,
for `libnmstate.show()` and `nmstatectl show`, routes next hop to
ignored interface is now marked with `state: ignore` which will be
ignored during apply action.

Integration test cases included.

Resolves: https://issues.redhat.com/browse/RHEL-107130

Signed-off-by: Gris Ge <fge@redhat.com>