move gateway to option

[qinghon]

In some usage scenarios, the system gateway and dns are not required to be obtained, and the gateway implementation include HashMap, which is difficult to accept in some resource-constrained scenarios.
We can split the gateway part into a feature but add default_feature

[qinghon]

#102

[shellrow]

Thanks for the suggestion! and sorry for the late response.

Agree — in some environments, especially resource-constrained ones, the gateway and DNS information may not be necessary.

Splitting this functionality into a feature flag makes a lot of sense, and including it as a default-feature is a great way to minimize breaking changes while still offering a more lightweight build option for those who need it.

I'm currently planning some changes to the Interface struct and its implementation for the next release, and I intend to include your PR(#102) as part of that update.