oonimkall: mobile api for running WebConnectivity

[bassosimone]

Yesterday we had a team meeting where we discussed the importance
of stopping using MK wrappers for running experiments.

Here's a PoC showing how we can do that for WebConnectivity. It
seems to me we probably want to add more tests and merge this code
such that we can experiment with it quite soon.

There seems to be opportunities for auto-generating code, BTW.

While working on this PoC, I've also took a chance to revamp the
external documentation of pkg/oonimkall.

Reference issue: ooni/probe#1346

[bassosimone]

For reference, this is the generated Java API:

public class WebConnectivityConfig {
  public WebConnectivityConfig();
  public void setCallbacks(ExperimentCallbacks v);
  public void setInput(String v);
}

public class WebConnectivityResults {
  public WebConnectivityResults();
  public double getKibiBytesReceived();
  public double getKibiBytesSent();
  public String getMeasurement();
}

public class Session {
  // ... other methods ...

  public WebConnectivityResults webConnectivity(Context ctx, WebConnectivityConfig config) throws Exception;
}