Streamline complementary REST requests

There are at least two use cases for developers to make additional REST API calls beyond what is handled already by the SDK:

1. During discovery - The tap may want to ping a REST API ahead of calls to get_records().
2. During data-append - The tap may need to supliment the core data with additional calls in the post_process() handler.

For both of these cases, we should establish a pattern that developers can implement with confidence.

As of today, the developer would likely implement calls against the requests library directly. The biggest downside of this approach is that those extra calls may have inconsistencies in regards to authentication and retry logic.