Support for the Subaccount API

Implemented all the Subaccount API routes for the Paystack client. The following operations are now possible:

• Create a subaccount
• List all subaccounts
• Fetch a subaccount based on ID or subaccount code
• Updated a subaccount based on ID or subaccount code

What's Changed

• Implemented the Transaction Split API by @morukele in #9
• Customer api implementation by @morukele in #11
• Preparation for Transaction-Split-Test by @morukele in #12
• Merge pull request #12 from morukele/main by @morukele in #13
• Subaccount api by @morukele in #14

Full Changelog: v0.1.6...v0.2.0