feat: support non-primary reference keys of source and target models for hasManyThrough

[aaqilniz]

Loopback offers the hasManyThrough relation with keysFrom & keysTo using primary keys only.
You can also define a hasManyThrough relation using non-primary but unique keys.
Like in the following example:

Customers
  id (PK)
  customerCode (UNIQUE)
  name

Product
  id (PK)
  sku (UNIQUE)
  name

Orders (through model)
  id (PK)
  customerCode (referencing customer)
  productSku     (referencing product)
  quantity

The product and customers are connected with Order not through primary keys but through unique keys.

This is how it looks:

// for product
@hasMany(() => Customer, {
    customReferenceKeyFrom: 'sku',
    customReferenceKeyTo: 'customerCode',
    through: {
      model: () => Order,
      keyFrom: 'productSku',
      keyTo: 'customerCode',
    },
  })
customers: Customer[];

Please note that this is backward compatible. There is no breaking change in the PR.

Checklist

• DCO (Developer Certificate of Origin) signed in all commits
• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

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[coveralls]

Pull Request Test Coverage Report for Build 21796125793

Details

• 0 of 4 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 54.821%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/repository/src/relations/has-many/has-many-through.helpers.ts	0	4	0.0%

Totals
Change from base Build 21725088232:	-0.03%
Covered Lines:	9572
Relevant Lines:	12459

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💛 - Coveralls

[aaqilniz]

Hi, @dhmlau. Can you please have a look at this PR?