Add a function for subnetting by requested rules

[yunkon-kim]

• Add models for subnetting request including CIDR block and rules
• Add SubnettingBy() and related functions
• Add GetName() which is missing member method
• (Minor) Relocate or rename some existing functions

Related to #1412

This PR will add a user-friendly function for subnetting a CIDR block by request rules.
I think it's helpful to make a draft of a global muti-cloud network configuration, like the "global base network - VPCs - subnets" style.
Note - By adding a rule, more than 3 depths are possible.

Command

./netutil -c "10.0.0.0/16" -s 16 -n 500

Example (line 198-218)

In this example, a CIDR Block is divided into multiple subnets by a rule {Type: "minSubnets", Value: 16}.
Each subnet is divided again into multiple subnets by a rule {Type: "minHosts", Value: 500}.

	fmt.Println("\nSubnetting a CIDR block by requests")
	request := netutil.SubnettingRequest{
		CIDRBlock: cidrBlock,
		SubnettingRules: []netutil.SubnettingRule{
			{Type: "minSubnets", Value: minSubnets},
			{Type: "minHosts", Value: hostsPerSubnet},
		},
	}

	// Subnetting by requests
	networkConfig, err := netutil.SubnettingBy(request)
	if err != nil {
		fmt.Println("Error subnetting network:", err)
		return
	}

	pretty, err = json.MarshalIndent(networkConfig, "", "   ")
	if err != nil {
		fmt.Printf("marshaling error: %s\n", err)
	}
	fmt.Printf("[Subnetting result]\n%s\n", string(pretty))

[Subnetting result]

{
   "cidrBlock": "10.0.0.0/16",
   "subnets": [
      {
         "cidrBlock": "10.0.0.0/20",
         "subnets": [
            {
               "cidrBlock": "10.0.0.0/23"
            },
            {
               "cidrBlock": "10.0.2.0/23"
            },
            {
               "cidrBlock": "10.0.4.0/23"
            },
            {
               "cidrBlock": "10.0.6.0/23"
            },
            {
               "cidrBlock": "10.0.8.0/23"
            },
            {
               "cidrBlock": "10.0.10.0/23"
            },
            {
               "cidrBlock": "10.0.12.0/23"
            },
            {
               "cidrBlock": "10.0.14.0/23"
            }
         ]
      },
      {
         "cidrBlock": "10.0.16.0/20",
         "subnets": [
            {
               "cidrBlock": "10.0.16.0/23"
            },
            {
               "cidrBlock": "10.0.18.0/23"
            },
            {
               "cidrBlock": "10.0.20.0/23"
            },
            {
               "cidrBlock": "10.0.22.0/23"
            },
            {
               "cidrBlock": "10.0.24.0/23"
            },
            {
               "cidrBlock": "10.0.26.0/23"
            },
            {
               "cidrBlock": "10.0.28.0/23"
            },
            {
               "cidrBlock": "10.0.30.0/23"
            }
         ]
      },
      ...
   ]
}

This is the same result as #1417 (except name field).

[yunkon-kim]

Additionally, considering this PR, it seems to provide the basic functions of netutil.

Resolve #1412

[seokho-son]

/approve

Thanks!! :)