Experiment with moving serialization out of Filter classes

[solebared]

Why

This allows us to extract serialization out of the Filter classes, leaving it appropriately in the controller -> serialization layer.

What

Introduces a FilterGroupBlueprint which dynamically determines whether to use a regular FilterOptionBlueprint or the special case ContributionTypeFilterOptionBlueprint.

Testing

Covered by existing tests.

Outstanding Questions, Concerns and Other Notes

?

Meta

@h-m-m , this came out of a pairing discussion we had many months ago. I remember spiking this out and found the branch lying around. Figured i'd bring it up to date and put up a PR. Totally happy to close this out though if this direction doesn't feel right.

Pre-Merge Checklist

• [x] Security & accessibility have been considered
• [x] Tests have been added, or an explanation has been given why the features cannot be tested
• [x] Documentation and comments have been added to the codebase where required
• [x] Entry added to CHANGELOG.md if appropriate
• [ ] Outstanding questions and concerns have been resolved
• [ ] Any next steps have been turned into Issues or Discussions as appropriate