Add Next/PreviousSubnet() to netutil pkg

[yunkon-kim]

• Add Next/PreviousSubnet()
• Add these examples

This PR will add NextSubent() and PreviousSubent() and the following examples.

NextSubnet() test example - In case 4, Error: the next subnet is outside the base network range

NextSubnet() test example
[NextSubnet() Case1] Base Network CIDR: 10.0.0.0/16, Current Subnet CIDR: 10.0.0.0/18
 Next Subnet: 10.0.64.0/18
[NextSubnet() Case2] Base Network CIDR: 10.0.0.0/16, Current Subnet CIDR: 10.0.64.0/18
 Next Subnet: 10.0.128.0/18
[NextSubnet() Case3] Base Network CIDR: 10.0.0.0/16, Current Subnet CIDR: 10.0.128.0/18
 Next Subnet: 10.0.192.0/18
[NextSubnet() Case4] Base Network CIDR: 10.0.0.0/16, Current Subnet CIDR: 10.0.192.0/18
 Error: the next subnet is outside the base network range

PreviousSubnet() test example - In case 1, Error: the previous subnet is outside the base network range

PreviousSubnet() test example
[PreviousSubnet() Case1] Base Network CIDR: 10.0.0.0/16, Current Subnet CIDR: 10.0.0.0/18
 Error: the previous subnet is outside the base network range
[PreviousSubnet() Case2] Base Network CIDR: 10.0.0.0/16, Current Subnet CIDR: 10.0.64.0/18
 Previous Subnet: 10.0.0.0/18
[PreviousSubnet() Case3] Base Network CIDR: 10.0.0.0/16, Current Subnet CIDR: 10.0.128.0/18
 Previous Subnet: 10.0.64.0/18
[PreviousSubnet() Case4] Base Network CIDR: 10.0.0.0/16, Current Subnet CIDR: 10.0.192.0/18
 Previous Subnet: 10.0.128.0/18

[seokho-son]

/approve

Thanks! :)