feat(core): add expr helper to allow custom expressions in EM API

Allows to get around strict `FilterQuery` typing when we need to use
some SQL functions or expressions.

```ts
em.find(Book2, { [expr('upper(title)')]: ['B1', 'B2'] }); // no TS errors
```

Related: #802

packages/core/src/utils/QueryHelper.ts

@@ -199,3 +199,5 @@ export class QueryHelper {
   }
 
 }
+
+export const expr = (sql: string) => sql;

tests/EntityManager.postgre.test.ts

@@ -1,6 +1,6 @@
 import { v4 } from 'uuid';
 import {
-  Collection, Configuration, EntityManager, LockMode, MikroORM, QueryFlag, QueryOrder, Reference, Utils, Logger, ValidationError, wrap, UniqueConstraintViolationException,
+  Collection, Configuration, EntityManager, LockMode, MikroORM, QueryFlag, QueryOrder, Reference, Utils, Logger, ValidationError, wrap, expr, UniqueConstraintViolationException,
   TableNotFoundException, NotNullConstraintViolationException, TableExistsException, SyntaxErrorException, NonUniqueFieldNameException, InvalidFieldNameException,
 } from '@mikro-orm/core';
 import { PostgreSqlDriver, PostgreSqlConnection } from '@mikro-orm/postgresql';
@@ -1151,6 +1151,18 @@ describe('EntityManagerPostgre', () => {
     expect(test.id).toBeDefined();
   });
 
+  test('find with custom function', async () => {
+    const author = new Author2('name', 'email');
+    const b1 = new Book2('b1', author);
+    const b2 = new Book2('b2', author);
+    const b3 = new Book2('b3', author);
+    await orm.em.persistAndFlush([b1, b2, b3]);
+    orm.em.clear();
+
+    const books = await orm.em.find(Book2, { [expr('upper(title)')]: ['B1', 'B2'] });
+    expect(books).toHaveLength(2);
+  });
+
   test('find by joined property', async () => {
     const author = new Author2('Jon Snow', 'snow@wall.st');
     const book1 = new Book2('My Life on The Wall, part 1', author);