feat(core): add support for virtual entities

[B4nan]

Virtual entities don't represent any database table. Instead, they dynamically resolve to an SQL query (or an aggregation in mongo), allowing to map any kind of results onto an entity. Such entities are mean for read purposes, they don't have a primary key and therefore cannot be tracked for changes. In a sense they are similar to (currently unsupported) database views.

To define a virtual entity, provide an expression, either as a string (SQL query):

@Entity({
  expression: 'select name, age, ' +
    '(select count(*) from book b where b.author_id = a.id) as total_books, ' +
    '(select group_concat(distinct t.name) from book b ' +
      'join tags_ordered bt on bt.book_id = b.id ' +
      'join book_tag t on t.id = bt.book_tag_id ' +
      'where b.author_id = a.id ' +
      'group by b.author_id) as used_tags ' +
    'from author a group by a.id',
})
export class BookWithAuthor {

  @Property()
  title!: string;

  @Property()
  authorName!: string;

  @Property()
  tags!: string[];

}

Closes #1104

[codecov-commenter]

Codecov Report

Merging #3351 (91adf04) into master (eff463c) will decrease coverage by 0.00%.
The diff coverage is 98.43%.

@@             Coverage Diff             @@
##            master    #3351      +/-   ##
===========================================
- Coverage   100.00%   99.99%   -0.01%     
===========================================
  Files          203      205       +2     
  Lines        12456    12588     +132     
  Branches      2872     2905      +33     
===========================================
+ Hits         12456    12587     +131     
- Misses           0        1       +1     

Impacted Files	Coverage Δ
packages/core/src/decorators/Entity.ts	100.00% <ø> (ø)
packages/core/src/drivers/DatabaseDriver.ts	100.00% <ø> (ø)
packages/core/src/drivers/IDatabaseDriver.ts	100.00% <ø> (ø)
...ges/entity-generator/src/EntitySchemaSourceFile.ts	100.00% <ø> (ø)
packages/knex/src/AbstractSqlDriver.ts	99.78% <95.45%> (-0.22%)	⬇️
packages/core/src/EntityManager.ts	100.00% <100.00%> (ø)
packages/core/src/entity/EntityFactory.ts	100.00% <100.00%> (ø)
packages/core/src/hydration/ObjectHydrator.ts	100.00% <100.00%> (ø)
packages/core/src/metadata/MetadataStorage.ts	100.00% <100.00%> (ø)
packages/core/src/metadata/MetadataValidator.ts	100.00% <100.00%> (ø)
... and 21 more

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