Handle global and product specific environment variables

[raybesiga]

Some of the variables required to initialize the library are global, while others are product specific.

The global variables are;

• Base URL (Sandbox or Production URL)
• Platform Environment (Sandbox or Production)
• Callback Host

The product specific ones are;

• Subscription Key (Primary or Secondary)
• User ID
• User/API Secret

Describe the solution you'd like

We should initialize the library with global variables and then initialize the product clients with product-specific variables.