Allow custom implementation of API Request Handler

[dkarzon]

Added in IApiRequestHandler so that custom logic can be applied to the underlying API request handling by integrators.
To do this I also added the IApiRequestHandlerFactory interface which will create an instance of IApiRequestHandler with a default implementation ApiRequestHandlerFactory that will return the existing ApiRequestHandler with the existing parameters.
This allows the underlying HTTP calls to be overridden by the integrator for extra custom functionality (with the idea that these implementations will be based on the existing implementations).

Example usages of this interface:

• Adding custom logging to outgoing HTTP requests
• Implementing retry logic within the HTTP handler for responses such as 429 too many requests or 504 gateway timeout