new: Add SiteType in regions

[yec-akamai]

📝 Description

This change exposes a new field SiteType in the regions related response. This field will be able to tell apart different site type.

✔️ How to Test

make ARGS="-run TestRegions_List" fixtures

Manual Test:

1. In a sandbox environment, run the following code:

import (
	"context"
	"os"

	"github.com/linode/linodego"
)

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

	regions, err := client.ListRegions(context.Background(), nil)
	if err != nil {
		panic(err)
	}

	println(regions[0].ID, regions[0].SiteType)
}

2. Observe the new field printed as expected.

[zliang-akamai]

LGTM! (tests passed of course)

[lgarber-akamai]

LGTM!