Support for the Subaccount API

[morukele]

We are happy to announce that the Paystack-rs crate now supports the Subaccount Route of the Paystack API.

You can now do the following:

• Create a subaccount
• Fetch a subaccount by account code or id
• List all subaccounts
• Modify a subaccount by account code or id

The methods are well documented. Feel free to open an Issue if you find any bugs!

Keep building!