Add console interceptor

The console interceptor prints track events to the console in real time.
To use it, enable it in the trace config like this:

data_sources: {
    config {
        name: "track_event"
        interceptor_config: {
            name: "console"
            enable_colors = true/false  # Automatic if omitted.
            output: STDOUT/STDERR       # STDOUT by default.
        }
    }
}

Screenshot: https://drive.google.com/file/d/1XGRAtOelOnvOTW2p-WPkWOYqy8QOGsof/view?usp=sharing

Bug: 170628040
Change-Id: I303cd391883b26b4938180deb061ab0b1f285a70