Basic mocking server

[a-type]

Create an API mocking server that can serve user-defined mock payloads by matching against incoming requests and loading responses from storage.

Request matching logic should be written extensibly, and for this delivery it must only naively match the URL path.

For this delivery, mock responses are static values, defined by the user via a user interface.

Storage of configuration should be written extensibly, but the only implementation to be delivered right now is file-based.