net: add ability to customize gateway address

[Guilospanck]

This allows the possibility of specifying the gateway address when starting colima.

Issue (#1434)

When connected to a VPN that has the same Gateway (192.168.5.2) as colima/lima, I couldn't reach out to services in the other side from inside the container. Guess is the conflict of addresses.

• VPN has the same gateway address as colima (192.168.5.2);
• Tries to pull docker image from private registry only accessible via the VPN, which needs to pass through the gateway;
• Not possible.

Proposed solution

Add a new flag/config to allow specifying the gateway address you want colima to have. If none is passed, it defaults to 192.168.5.2. This prevents the conflicts as above. In my testings it worked for my situation and for other parts that don't depend on the VPN as well.

The gateway must be a valid IPv4 and terminated with 2.

[abiosoft]

I've been away for a while but I am ready to get this merged :)
Kindly attend to the comments.

[Guilospanck]

@abiosoft Improved it based on your comments

[abiosoft]

Thanks :)