Wrap function calling including error handling in Calls API

[thekid]

In essence, this PR simplifies function calling by handling JSON de- and encoding as well as supplying a default error handling mechanism:

Before

try {
  $return= json_encode($functions->invoke(
    $call['function']['name'],
    json_decode($call['function']['arguments'], true)
  );
} catch (Throwable $t) {
  $t->printStackTrace();
  $return= json_encode(['error' => nameof($t), 'message' => $t->getMessage()]);
}

After

// Setup calls
$calls= $functions->calls()->catching(fn($t) => $t->printStackTrace());

// Call the function, including handling JSON de- and encoding and converting
// caught exceptions to a serializable form.
$return= $calls->call($call['function']['name'], $call['function']['arguments']);

[thekid]

Released in https://github.com/xp-forge/openai/releases/tag/v0.5.0