Feedback on Relation Inputs Plugin

Would love some feedback on efforts to port the nested-mutations-plugin form v4 to v5.

https://github.com/litewarp/graphile-relation-inputs-plugin

Thanks again for all your hard work.

- [X] am interested in building this feature myself
- [X] am interested in collaborating on building this feature
- [X] am willing to help testing this feature before it's released
- [X] am willing to write a test-driven test suite for this feature (before it exists)
- [ ] am a [Graphile sponsor](https://www.graphile.org/sponsor/) ❤️
- [ ] have an active [support or consultancy contract](https://www.graphile.org/support/) with Graphile