Windows multiple default gateway. Fixes #32

[fireflycons]

• Select gateway with lowest metric if multiple default routes present (should I do this for the others too?)
• Enhance tests to assert errors
  • Create concrete type for each error
  • Assert it is returned in tests

[fireflycons]

FYI My interest in this package is here: https://registry.terraform.io/providers/fireflycons/localos/latest/docs/data-sources/private_ip