fix: parse subnet cidr and calculate gateway

[rbtr]

Reason for Change:

Prior to this change, we parsed the subnet string into a byte array as [1].[2].[3].[4], then built the gateway address for our netns by dropping [4] and replacing it with .1, as [1].[2].[3].1. This doesn't work for subnets that begin on a non-zero final octect, such as 10.0.0.128/27, which should have a gateway address of 10.0.0.129.

This change updates the relevant code to calculate the gateway address as the first available IP in the provided subnet, fixing these bugs.

Along the way, it also:

• fixes the log of the response body to print the actual response string instead of the pointer to the body
• returns an error if the CIDR does not have capacity to calculate a gateway IP (like if it is a /32)

Issue Fixed:

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests

Notes: