Request/Response Mocks Defined As OpenAPI Examples

**Issue**
A present Open API 3.0 Examples are returned randomly, the only differentiator is setting a Preferred HTTP response code:
```
// Choose a random example to return.
keys := make([]string, 0, len(mt.Examples))
for k := range mt.Examples {
keys = append(keys, k)
}
selected := keys[rand.Intn(len(keys))]
return mt.Examples[selected].Value.Value, nil
```

**Request**
It would be great if we could define which mocks to return also using path params, query strings, and even request/response payloads. See how Microcks uses this ability: http://microcks.github.io/using/openapi/

Start by reviewing the random selection logic shown for mt.Examples and the OpenAPI 3.0 example behavior described in the issue. Compare the requested path-parameter, query-string, and payload matching with the linked Microcks reference. Done means mocks can be selected deterministically from those request and response details instead of only by random choice.

Written by the indexing model from the issue text.