[10.x] Add lowercaseKeys and uppercaseKeys methods to Arr helper

[ilbeygi]

This pull request introduces two new methods to the Arr helper in Laravel, namely lowercaseKeys and uppercaseKeys. These methods allow users to conveniently convert all keys in an associative array to lowercase or uppercase, providing a more versatile and user-friendly array manipulation utility.

Changes :

• Added lowercaseKeys method to convert array keys to lowercase.
• Added uppercaseKeys method to convert array keys to uppercase.

// Convert keys to lowercase
$lowercasedArray = Arr::lowercaseKeys($originalArray);

// Convert keys to uppercase
$uppercasedArray = Arr::uppercaseKeys($originalArray);

These additions enhance the functionality of the Arr helper and provide users with a consistent way to handle array keys in different casing scenarios.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If possible, please consider releasing your code as a package so that the community can still take advantage of your contributions!

If you feel absolutely certain that this code corrects a bug in the framework, please "@" mention me in a follow-up comment with further explanation so that GitHub will send me a notification of your response.