This is an alternative for #27407 with a solution based on gencodec.
With the PR, one can now configure like this:
```
# config.toml
[Node.P2P]
NAT = "extip:33.33.33.33"
```
```shell
$ geth --config config.toml
...
INFO [01-17|16:37:31.436] Started P2P networking self=enode://2290...ab@33.33.33.33:30303
```
Felix Lange