fix: reuse foreign key field in @belongsTo transformer

[edwardfoyle]

Description of changes

Modifies the @belongsTo transformer to reuse the foreign key field created by the corresponding @hasMany directive when generating the belongsTo resolvers and types. It also reuses the same field name when the corresponding relation is @hasOne. While this is not strictly necessary, it creates a more consistent naming experience. It also means that a @hasOne relation can be changed to a @hasMany relation and existing relationships will be maintained.

Also created an E2E test utility that wraps a bunch of common logic for spinning up test APIs. Hopefully this can be used across many tests in the future

Issue #, if available

Description of how you validated changes

Manually tested and added an E2E test

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[cjihrig]

LGTM, thanks.