Introduce HigherOrderWhenProxy

[netpok]

I propose we introduce proxies for when on collections and the query builder.

Example use cases:

• Collection sorting: $collection->when($field)->sortBy($field)
• Database filtering: User::when($coreDataOnly)->select(['name', 'email'])->get()

Working principle:

• if the value is true, then the called method is called on the target object and the result is returned
• if the value is false, then the called method is skipped and the target is returned

[driesvints]

Closing as this idea was rejected by Taylor.

[netpok]

This feature was implemented in 9.x via #37632