Add support for sending Chrome DevTools commands

[aboks]

ChromeDriver has two endpoints for calling the DevTools API. This PR adds support for calling these endpoints (sending DevTools commands) using php-webdriver.

[aboks]

@OndraM is there anything I could do to help move this PR forward?

[OndraM]

After a further consideration, I think this may need a different approach. Lets wait with finishing it after 1.9.0.

However executing ChromeDevTools protocol is possible since php-webdriver 1.8.0 via custom commands:

Send command:

$driver->executeCustomCommand(
    '/session/:sessionId/goog/cdp/execute', 
    'POST',
    ['cmd' => 'Performance.enable']
);

Send command and receive result:

$metrics = $driver->executeCustomCommand(
    '/session/:sessionId/goog/cdp/execute',
    'POST',
    ['cmd' => 'Performance.getMetrics']
);

Send command with parameters and receive result:

$return = $driver->executeCustomCommand(
    '/session/:sessionId/goog/cdp/execute',
    'POST',
    ['cmd' => 'Runtime.evaluate', 'params' => ['expression' => 'window.location.toString()']]
);

[OndraM]

Also the endpoint URL was meanwhile changed in chromium to /session/:sessionId/goog/cdp/execute

https://bugs.chromium.org/p/chromedriver/issues/detail?id=2307

[OndraM]

Replaced by #832 . Thanks @aboks for this initial implementation!

[OndraM]

✔️ This feature is now merged and documented in our wiki with examples.