Update the SmartDateRecurTrait to make arbitrary subsets easier to extract

Problem/Motivation

I have the need to programmatically render the next x instances of a recurring rule
I ended up filtering them manually in a request, but it might make more sense as an api the module can review.

@mandclu mentioned this may be a good initial issue for a 5.x version of Smart date.

Steps to reproduce

N/A

Proposed resolution

subsetInstances should likely return the instances instead of a render array

Remaining tasks

Agree on plan
Build MR

User interface changes

API changes

Data model changes