[Site Design Revamp] Add optional group array for remote site designs

[twstokes]

Description

Related:

• [Site Design Revamp] Main View - Add recommended designs section WordPress-iOS#18434
• [Site Design Revamp] Main View - Add recommended designs section WordPress-iOS#18657
• D80005-code

This PR adds the ability to decode the optional group array parameter from remote designs served by the themes endpoint. It is backwards compatible in the event that the API doesn't serve the group parameter.

Testing Details

Use wordpress-mobile/WordPress-iOS#18657 for testing.

• Please check here if your pull request includes additional test coverage.
• I have considered updating the version in the .podspec file.

[Gio2018]

This looks good to me @twstokes !
Let's just make sure, before we cut the release, that this won't break anything in trunk. It shouldn't, but some extra caution is probably better since we are working on a feature branch.

[twstokes]

This looks good to me @twstokes ! Let's just make sure, before we cut the release, that this won't break anything in trunk. It shouldn't, but some extra caution is probably better since we are working on a feature branch.

Sounds good @Gio2018. 👍 I was thinking we'll do another PR pointed to trunk when we're ready to merge the feature branch, so we can take that opportunity to double-check things.