feat(core): add support for polymorphic embeddables

[B4nan]

Polymorphic embeddables allow us to define multiple classes for a single embedded property and the right one will be used based on the discriminator column, similar to how single table inheritance work.

enum AnimalType {
  CAT,
  DOG,
}

@Embeddable({ abstract: true, discriminatorColumn: 'type' })
abstract class Animal {

  @Enum(() => AnimalType)
  type!: AnimalType;

  @Property()
  name!: string;

}

@Embeddable({ discriminatorValue: AnimalType.CAT })
class Cat extends Animal {

  @Property({ nullable: true })
  canMeow?: boolean = true;

  constructor(name: string) {
    super();
    this.type = AnimalType.CAT;
    this.name = name;
  }

}

@Embeddable({ discriminatorValue: AnimalType.DOG })
class Dog extends Animal {

  @Property({ nullable: true })
  canBark?: boolean = true;

  constructor(name: string) {
    super();
    this.type = AnimalType.DOG;
    this.name = name;
  }

}

@Entity()
class Owner {

  @PrimaryKey()
  id!: number;

  @Property()
  name!: string;

  @Embedded(() => [Cat, Dog])
  pet!: Cat | Dog;

}

Closes #1165

BREAKING CHANGE:

Embeddable instances are now created via EntityFactory and they respect the
forceEntityConstructor configuration. Due to this we need to have EM instance
when assigning to embedded properties.

Using em.assign() should be preferred to get around this.

Deep assigning of child entities now works by default based on the presence of PKs in the payload.
This behaviour can be disable via updateByPrimaryKey: false in the assign options.

mergeObjects option is now enabled by default.

[codecov-commenter]

Codecov Report

Merging #2426 (fd57049) into master (3fca9a6) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2426   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          188       188           
  Lines        10640     10702   +62     
  Branches      2415      2436   +21     
=========================================
+ Hits         10640     10702   +62     

Impacted Files	Coverage Δ
packages/core/src/decorators/Embedded.ts	100.00% <ø> (ø)
packages/core/src/decorators/Entity.ts	100.00% <ø> (ø)
packages/core/src/typings.ts	100.00% <ø> (ø)
packages/core/src/decorators/Embeddable.ts	100.00% <100.00%> (ø)
packages/core/src/entity/EntityAssigner.ts	100.00% <100.00%> (ø)
packages/core/src/entity/EntityFactory.ts	100.00% <100.00%> (ø)
packages/core/src/errors.ts	100.00% <100.00%> (ø)
packages/core/src/hydration/ObjectHydrator.ts	100.00% <100.00%> (ø)
packages/core/src/metadata/EntitySchema.ts	100.00% <100.00%> (ø)
packages/core/src/metadata/MetadataDiscovery.ts	100.00% <100.00%> (ø)
... and 9 more

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