L-2183 Ensure flushed logs in long running scripts

[PetrHeinz]

Resolves #11, resolves #21

Adds new option flushIntervalMs for LogtailHandler which ensures that logs are sent at least every X (every 5 seconds by default). This assumes that logs are created somewhat regularly.

In default configuration, long running script would eventually run out of memory. For this reason, I've changed default bufferLimit to 1000 and changed default behavior from dropping logs to flushing logs on overflow. Also introduced throwExceptions option to control whether we should throw RuntimeException on curl failure, which is by default false now to prevent apps from crashing.

Since we have a bit too many parameters at this point, I've created a builder for comfortable handler creation (all methods will be suggested by IDE, all options named nicely). As an example, see comparison of suggested config from #21:

$logger = new Logger("example-app");
$logger->pushHandler(
      new LogtailHandler(
          "$SOURCE_TOKEN",
          $this->log_level,
          true,
          \Logtail\Monolog\LogtailClient::URL,
          5, //will send logs to Better Stack every fifth log record. You can adjust this to minimise traffic if your script produces lot of messages.
          true //send messages instead of removing old messages
      ));

$logger = new Logger("example-app");
$handler = LogtailHandlerBuilder::withSourceToken("$SOURCE_TOKEN")
  ->withLevel($this->log_level)
  ->withFlushOnOverflow(true)
  ->withBufferLimit(5)
  ->build();
$logger->pushHandler($handler);

[PetrHeinz]

Planning on releasing this as 2.2.0 and 3.2.0 (after adding static types and merging into main branch).

I've added a note into our PHP logging docs about terminating scripts with CTRL+C, but made no effort on implementing it into our library (it doesn't seem to be part of our responsibility).

I will update the code snippets to use LogtailHandlerBuilder instead after release.

[curusarn]

Thank you! Left on comment.