Allow importing HAR files to build mock responses

[waldekmastykarz]

Right now, in order to mock Graph API responses with the Proxy, you need to manually build the responses file. We should allow creating responses from a HAR file, to simplify the process. To start, we'd support HAR files that are generated by Chromium-based browsers (Edge, Chrome) and consider extending to other sources in the future.

Spec:

• introduce new option --har-file that customers can use to point the proxy to a .har file
• when customer uses the --har-file options, the proxy starts, generates mock responses and closes
• generated responses are sent to stdout. That way customers can decide themselves how they want to handle them: pipe to another command or save to a file using output redirection > responses.json

[waldekmastykarz]

Duplicate of #308