Deprecation notice with php 8.1 : return type incompatibility

[monnardj]

In PHP 8.1, a new return type compatibility rule has been introduced for interfaces. Several methods of MultiRequestSubResult class in /local/kaltura/API/KalturaClientBase.php class do not respect the signature expected by the ArrayAccess interface.

This produces notices when they are called, such as :
Deprecated: Return type of MultiRequestSubResult::offsetExists($offset) should either be compatible with ArrayAccess::offsetExists(mixed $offset): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in .../KalturaClientBase.php on line 51

In future versions of php, these notices might become fatal errors. Thus, the method definitions offsetExists, offsetGet, offsetSet, offsetUnset should be adapted according to the definition of ArrayAccess interface in https://www.php.net/manual/en/class.arrayaccess.php.