Implement Card Vault Payment

Implement Card Vault Payment (#3)

* Implement Card Vault Payment

Test was done differently from existing tests to remove dependency on
the staging Paymaya API.

* Make the Payment request object have fluent setters

* Import JsonSerializable for consistency with existing classes

Fixes nullable fields

2.0.1

Add nullable to non-required fields

2.0.0

Major change in directory structure and class names