new: Update VPCSubnet linodes field to reflect API changes

[lgarber-akamai]

📝 Description

This PR updates the VPCSubnet.Linodes field to reflect the new structure returned from the API. This structure now includes an interfaces field that contains related Linode interfaces for this VPC.

Additionally, this change adds a VPC test resource modifier to keep regions consistent in E2E tests.

✔️ How to Test

E2E Testing:

make ARGS="-run TestVPC_Subnet_WithInstance" fixtures

Manual Testing:

Inside of a linodego sandbox environment (dx-devenv), write and run the following example in main.go:

package main

import (
	"context"
	"fmt"
	"github.com/linode/linodego"
	"os"
)

func main() {
	client := linodego.NewClient(nil)
	client.SetToken(os.Getenv("LINODE_TOKEN"))

	vpc, _ := client.CreateVPC(context.Background(), linodego.VPCCreateOptions{
		Label:  "foobar",
		Region: "us-east",
		Subnets: []linodego.VPCSubnetCreateOptions{
			{
				Label: "foobar",
			},
		},
	})

	inst, _ := client.CreateInstance(context.Background(), linodego.InstanceCreateOptions{
		Label:    "foobar",
		Type:     "g6-nanode-1",
		Region:   "us-east",
		Image:    "linode/alpine3.18",
		RootPass: "v3ryl3g1tp4ssw0rd!!!!!!!!",
		Interfaces: []linodego.InstanceConfigInterfaceCreateOptions{
			{
				Purpose: linodego.InterfacePurposePublic,
			},
			{
				Purpose:  linodego.InterfacePurposeVPC,
				SubnetID: &vpc.Subnets[0].ID,
			},
		},
	})

	subnet, _ := client.GetVPCSubnet(context.Background(), vpc.ID, vpc.Subnets[0].ID)

	fmt.Println(subnet.Linodes)
}

[amisiorek-akamai]

Tests work on my end! One small thing, maybe we should change "us-east" to either "us-mia" or "us-lax" in TestVPC_Subnet_WithInstance if it's been moved to prod? I don't think it's available in us-east yet

[lgarber-akamai]

Tests work on my end! One small thing, maybe we should change "us-east" to either "us-mia" or "us-lax" in TestVPC_Subnet_WithInstance if it's been moved to prod? I don't think it's available in us-east yet

@amisiorek-akamai Nice catch! I'll update the test case to reflect VPC availability in prod

[jriddle-linode]

LGTM!