Add strongly typed response for listing variants by type

[DiegoFaFe]

Motivation

Which issue does this fix? Fixes #146

If no issue exists, what is the fix or new feature? Were there any reasons to fix/implement things that are not obvious?

Added the method to return strongly typed variants when filtering by type.

Checklist

• Code follows coding conventions held in this repo
• Automated tests have been added
• Tests are passing
• Docs have been updated (if applicable)
• Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

If manual testing is required, what are the steps?

[pokornyd]

hello Diego, thanks for the PR. I'll review the proposed changes and come back to you with any remarks, thanks for your patience.

[pokornyd]

I took the liberty of solution-wide refactoring of some null checks and unnecessary lambdas. the method implementation and tests are in line with the rest of the code, approving and merging