Testing field-level encryption requires a mock KMS server. This enables us to test with fine-grained control.

• Add a way to configure with canned responses.
• Add a mechanism to configure an error response, ala configureFailPoint with errors that could possibly be returned by a real KMS server. For convenience of testing, it may be preferable to also listen for the MongoDB wire protocol (on another port) using mockupdb. This way, drivers can configure mock KMS without needing to speak HTTP.