Enhancement: define failure rate for each mocked response

### Background

Currently, I am researching if we may use the ms-dev-proxy as a tool to support us in integration/manual/QA tests for our apps, and mocking responses is just the perfect functionality we could use 😉.

### Idea

1. Currently the ms-dev-proxy will look for `responses.json` file in the working directory which is just perfect to keep mock related to the app in the projects folder. What I was thinking is maybe ms-dev-proxy could have a new option that would allow specifying the relative path to the `response.json` with mocks to be used. Maybe something like `-r --responses-file-path` (optional). The aim for this would be to have different test scenarios with different mocks for the same app and then use them as part of integration tests. That way I could keep something like
```
.
├── MyApp
├── TestScenarios
│ ├── groups-fail-timeout-responses.json
│ └── throttle-responses.json
```

In this case, each integration test could start the ms-dev-proxy giving different `responses.json` mocks as param.

TBH this is very low 😉 (and probably stupid) idea as the current workaround I have for it is:
- keep `responses.json` in subfolders and run the ms-dev-proxy in the subfolder with correct mocks as working directory
- the test may just copy/paste the needed `responses.json` file to be used for this test.

2. Second idea I had (sorry for adding 2 ideas in one issue, I am a bit lazy and running out of time 😝), is to give possibility to define the failure rate for each mocked response in case we are mocking error response.