Add support for subaccount management APIs

[vmj]

• Listing accounts: https://developers.upcloud.com/1.3/3-accounts/#get-account-list
  • UpCloudApi.accounts(...): GET /1.3/account/list -> 200 OK
  • List<Account> response.accounts
  • Account is a new Resource
    • List<String> roles
    • String type
    • String username
• Get account details: https://developers.upcloud.com/1.3/3-accounts/#get-account-details
  • AccountApi (new API trait): HTTP and username
    • AccountApi.load(): GET /1.3/account/details/username (would "sub" instead of "details" work?)
    • Account response.account
      • A lot of simple (String) properties
      • Quite a few List<String> properties
      • Including type and username and roles from account listing
      • servers list adds a new property to Server resource: storage "yes" or "no"
      • servers list reuses uuid property
• Add subaccount: https://developers.upcloud.com/1.3/3-accounts/#add-subaccount
  • UpCloudApi.create(SubAccount, ...): POST /1.3/account/sub
    • NOTE: Takes SubAccount, not Account (they are very similar if not identical)
    • NOTE: POSTs it to /account/sub, not to /sub_account
• Modify subaccount: https://developers.upcloud.com/1.3/3-accounts/#modify-account-details
  • AccountApi.update(Account, ...): PUT /1.3/account/sub/username ("details" works too)
• Delete subaccount: https://developers.upcloud.com/1.3/3-accounts/#delete-subaccount
  • AccountApi.detele(...): DELETE /1.3/account/sub/username (would "details" work?)