PHP 8.1 compatibility

[ruudk]

When installing rdkafka from source (5.x branch) on PHP 8.1 RC6 I'm getting these deprecations:

#15 3.020 PHP Deprecated:  Return type of RdKafka\Metadata\Collection::count() should either be compatible with Countable::count(): int, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in Unknown on line 0
#15 3.020 PHP Deprecated:  Return type of RdKafka\Metadata\Collection::current() should either be compatible with Iterator::current(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in Unknown on line 0
#15 3.020 PHP Deprecated:  Return type of RdKafka\Metadata\Collection::next() should either be compatible with Iterator::next(): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in Unknown on line 0
#15 3.020 PHP Deprecated:  Return type of RdKafka\Metadata\Collection::key() should either be compatible with Iterator::key(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in Unknown on line 0
#15 3.020 PHP Deprecated:  Return type of RdKafka\Metadata\Collection::valid() should either be compatible with Iterator::valid(): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in Unknown on line 0
#15 3.020 PHP Deprecated:  Return type of RdKafka\Metadata\Collection::rewind() should either be compatible with Iterator::rewind(): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in Unknown on line 0

Please merge #472 so that we can try to fix this.