executable interface to openapi request/response validation

[karenetheridge]

On the command line, we can take a file (or piped from stdin) containing openapi document(s), and a serialized HTTP/1.1 request or response, and perform validation on it, providing error results just as we do for json-schema-eval in JSON::Schema::Modern.

Combined with #60, this opens the door to demonstrating an open standard for multiple implementations to follow for compliance testing.

In the future when #65 is in place, we can provide this data as well in order to demonstrate how requests and responses are parsed in the face of certain openapi definitions.