[7.x] Add whenKeyExists method

[emargareten]

This PR adds a method which allows filtering JsonResource to include only queried fields. e.g. when query has the select()/addSelect() methods or when specifying the columns in get() or paginate().

see #32662

[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.

[laserhybiz]

Implemented in #45376