Replace the hardcoded ip address with a dns lookup

[MabezDev]

I can't actually test that it works because my ISP is having issue right now 🙃.

I can test at least that the error handling works :D.

For my router I also needed to bump the number of DNS servers smoltcp has capacity for with:

# .cargo/config.toml
[env]
SMOLTCP_DNS_MAX_SERVER_COUNT = "3"

Be warned that you have to do a clean build for those changes to take effect, see: rust-lang/cargo#10358

[JurajSadel]

LGTM thank you! Also works fine!