fix: fix reference-style relationship validation

[palpatim]

Description of changes

This change fixes relationship validation for reference-style relationships to support two use cases:

1. Fixes support for multiple relationships between models.

Example:

type Person @model @auth(rules: [{allow: public, provider: iam}]) {
  name: String
  authoredPosts: [Post] @hasMany(references: ["authorId"])
  editedPosts: [Post] @hasMany(references: ["editorId"])
}

type Post @model @auth(rules: [{allow: public, provider: iam}]) {
  title: String!
  content: String!
  authorId: ID!
  author: Person @belongsTo(references: ["authorId"])
  editorId: ID
  editor: Person @belongsTo(references: ["editorId"])
}

2. Fixes support for recursive relationships

Example:

type TreeNode @model {
  value: String
  parentId: ID
  parent: TreeNode @belongsTo(references: ["parentId"])
  children: [TreeNode] @hasMany(references: ["parentId"])
}

Additional changes

• Add unit tests for previously missing validation cases
• Add utilities and associated unit tests for schema reflection

NOTE: New E2Es covering these cases will come in a followup PR

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• E2E tests pass
  • Separate run for rds_pg_userpool_auth passed except for yarn lock

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

[palpatim]

This is safe to do because we have validation elsewhere for empty associated fields. This check was causing an early return in recursive schemas.

[sundersc]

The changes look good to me. Do we have E2E tests with references style to make sure the runtime behavior works as expected for multiple and recursive relationships between types.

Update: Nvm, I missed your note on the E2E tests.