add split type and split value to subaccount.create service

[waptik]

Description

This PR aims at allowing merchants/devs to add the type of split payment and value they want during a subaccount creation which by default is a flat split payment type and a value of 0.

Sorry for all these files changes, i fetched and merged local files with upstream(this repo) which resulted in this.

Related Issue

https://github.com/Flutterwave/ravepay-nodejs/issues/52

Motivation and Context

The default type of split payment flat is great but sometimes we'd like to have a percentage based type and also set the amount/value we want when creating a new subaccount. This can be achieve using the dashboard.

How Has This Been Tested?

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] Refactor (non-breaking change which improves implementation)
• [ ] Performance (non-breaking change which improves performance. Please add associated performance test and results)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Non-functional change (xml comments/documentation/etc)

Checklist:

• [ ] My code follows the code style of this project.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.
• [ ] My branch follows the naming convention bug-<issue#>-<description> or feature-<issue#>-<description>

@Flutterwave/Corvus97 What do you think about these updates?