feat(logs): add log_flush_threshold flag to trigger auto flush

[Litarnus]

Adds an option to perform an threshold based flushing of buffered logs. By default, this options remains null and will only flush when the flush method is invoked.

Example:

\Sentry\init([
    'dsn' => '__YOUR_DSN__',
    'log_flush_threshold' => 2,
]);

\Sentry\logger()->info('test');
\Sentry\logger()->error('error'); // this line will trigger the flush

Note that the threshold will also apply if using the monolog handler since it funnels everything down to the LogsAggregator