Specify identifying columns on nested mutation upserts

[GavG]

• [x] Added or updated tests
• [ ] Documented user facing changes
• [x] Updated CHANGELOG.md

Changes

Adds optional identifyingColumns arg to upsert directive

Breaking changes

None

[GavG]

Thanks for the review @spawnia. Sorry I've been a bit quiet on this, recently had our first child so haven't had any free time 🙃

I've been giving the solution a bit more thought, I think supporting nested relationship upsert logic is a must, so I'll try and tackle that when I come to making the suggest changes.

I've also been considering my own use case again, where more complex upsert logic was required, and I think a cleaner solution to this might be to permit the specifying of a custom query class that would receive the input params in place of the package's query logic. I may have a go at this in a new fork later.