What Does This Do

Adds a mechanism for registering ad-hoc event listeners for okhttp requests.
A listener can be registered when building a request:

class MyListener extends OkHttpUtils.CustomListener {
    // override event handlers
} 

final Request request =
        new Request.Builder()
            .url(...)
            .post(...)
            .tag(OkHttpUtils.CustomListener.class, new MyListener())
            .build();

Motivation

This is needed for CI Visibility telemetry. There are metrics like requests count, errors count, request size, etc for events/coverages intake, CI Visibility Git API and other endpoitns.

Additional Notes

EventListener.NONE is what all the requests are using by default: it is a listener that has an empty handler for every event.
With the new mechanism this "empty" listener is replaced with a custom one for those requests that have the corresponding tag.

This PR only adds the means for registering custom listeners. Actual listeners will be added in a separate PR.

Jira ticket: CIVIS-2427