Add VPC Grant and Refactor `UserGrants` class

[zliang-akamai]

📝 Description

• Make UserGrants class serializable.
• Add VPC grant.
• Refactor code, including wrapping up some logic into properties and sort out imports.

✔️ How to Test

make testunit

import os
from linode_api4 import LinodeClient

client = LinodeClient(token=os.getenv('LINODE_TOKEN'))

user = client.account.user_create(
    "your.email@example.com",
    "yourtestusername",
)

print(vars(user.grants))
user.delete()