Support HAR export of HTTP logs

### Problem description

When using the `-l, --log-requests` flag, the CLI logs some basic information to the console about requests and responses, like method, URL and status code. It would be helpful to be able to record more information about requests and responses and analyse it in automated ways.

Support exporting HTTP logs in Tabris CLI to HAR. HAR is [commonly used in browsers and network analysis tools](http://www.softwareishard.com/blog/har-adopters/) for exchange of request and response data. Various tools exist that can visualize data from HAR archives, like e.g. [HAR-Analyzer](https://toolbox.googleapps.com/apps/har_analyzer/).

### Environment

- Tabris.js version: 3.5.0