Add Nightwatch::digest to proactively transmit events to the agent

[timacdonald]

Add a public facing Nightwatch::digest method that mimics the Pulse::digest method. It proactively transmits any pending events while also respecting sampling, if it has been configured.

This is useful in long running processes where you want to dynamically control when an ingest occurs.

Imagine:

• you are running two processes for redundancy
• only a single process does the work
• a cache lock is used to determine which process does the work

In this scenario, one process may do a bunch of work building up events and then the other process might step in and start doing a bunch of work. The events from process the first process may never be transmitted, as they are only transmitted once they reach the event threshold, which defaults to 500 events.

This PR allows fine-grained control over when events are ingested, e.g.,

use Laravel\Nightwatch\Facades\Nightwatch;

if (previously_was_main_process()) {
    // Proactively transmit events to the agent
    // rather than waiting to reach the event
    // threshold.
    Nightwatch::digest();
}

[timacdonald]

Exposing the finishExecution method doesn't make sense from a naming perspective. This isn't going to be used at the end of an execution, but mid-way through.

Nightwatch::digest() is much nicer.