[LB-Next] Support of Relation (BelongsTo & hasMany) for Repository and ModelDefinition

[dhmlau]

Details

Details can be found in #419.

For example, `Customer` and `Order` has the `belongsTo` relation. When customerId was given, 
it becomes the filter for the `Order` repository. It implies that `Order` repository is not a free 
form repository but is constrained by `customerID=xxx`.

When doing CRUD operation, make sure this constrained is honored.
Currently, we are using primary key/foreign key for the relationship. 
In LB Next, we should allow people to use any properties.

This task is only for belongsTo relation.

Acceptance Criteria

• Actual implementation on belongsTo and hasMany
• Have example in how to use it
• have test cases

BLOCKED BY

• Do we want a new Juggler? #537

[arash01]

I've created a new issue #812 and just realised it's duplicated.

[dhmlau]

Closing it in favor of #995