[12.x] Add Arr::soleValue method

[Amirhf1]

This PR adds a soleValue method to the Arr class. This method works similarly to sole, but instead of returning the entire item, it only returns a specific field or key from the sole item.

Since the sole method was recently added to the Arr class in version 12, and a similar soleValue method exists in Query Builder and Eloquent, adding this method to Arr creates greater consistency between Laravel's various APIs.

Changes:

• Add soleValue method to the Arr class
• Add comprehensive tests for the new functionality

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