Custom nested mutations using @cypher on input fields

[michaeldgraham]

This PR adds support for using the @cypher directive on INPUT_FIELD_DEFINITION, allowing for custom nested mutations that are translated using Cypher correlated subqueries, available in Neo4j version 4.1.

Initial documentation: https://deploy-preview-60--distracted-golick-08ed24.netlify.app/docs/guide-graphql-schema-design#custom-nested-mutations

Node Mutation API Tests

Schema: https://github.com/michaeldgraham/neo4j-graphql-js/blob/master/test/helpers/custom/testSchema.js

Tests: https://github.com/michaeldgraham/neo4j-graphql-js/blob/master/test/unit/custom/cypherTest.test.js

Experimental API

As of PR #531, when config.experimental = true, the arguments for generated node mutations have an input object format. Nested @cypher input fields also work for this more recent schema design option:

Schema: https://github.com/michaeldgraham/neo4j-graphql-js/blob/master/test/helpers/experimental/custom/testSchema.js

Tests: https://github.com/michaeldgraham/neo4j-graphql-js/blob/master/test/unit/experimental/custom/cypherTest.test.js

This PR seems relevant to #89, #331, #532, and #539.

The #532 feature request will be updated to include designs for follow-up enhancements to the augmentation process based on this PR.

The PR #543 updates the initial design for this feature.