API Generator: Add support for extending entities

[kaufmo]

Description

Add support for extending entities in API Generator.

Example

@Entity({ abstract: true })
export abstract class TimestampEntity extends BaseEntity {
    @Property({
        columnType: "timestamp with time zone",
    })
    @Field()
    createdAt: Date = new Date();

    @Property({ onUpdate: () => new Date(), columnType: "timestamp with time zone" })
    @Field()
    updatedAt: Date = new Date();
}

@Entity()
export class TestEntityWithTimestamps extends TimestampEntity {
    @PrimaryKey({ type: "uuid" })
    id: string = uuid();

    @Property()
    foo: string;
}

NOTE: We don't recommend inheritance for code reusage

Limits

This is just basic support, it will probably break when the base class involves root blocks or enums (where we depend on ts-morph)