Introduce ConnectionInterceptor for individual connection processing

[vtuan10]

Motivation

Currently, it is only possible to set static http header values once. It is not possible to add header values which change with each network request.
We have the need to add headers which change with each request, because we sign each request (request method, request body and some other meta information) with a timestamp in our app.

What changed

Similarly to Okhttp, this introduces an interceptor interface ConnectionInterceptor to process each connection, i.e. add custom header which change with each request. It can be set at the beginning and will be called for each network request.

Example

ConnectionInterceptor interceptor = new ConnectionInterceptor() {
    @Override public HttpURLConnection intercept(HttpURLConnection connection, byte[] body) {
        connection.setRequestProperty("Prop", new Date().toString() + " super secret signature");
        return connection;
    }
};
Countly.sharedInstance().setConnectionInterceptor(interceptor)