Support for operators in JSON columns

[evenfrost]

Describe the bug
Querying a JSON column with condition operators is not currently supported and results in a wrong query.

To Reproduce
Steps to reproduce the behavior:

// 'data' is JSON column and 'created_at' is nested field
await myRepository.count({
  data: {
    created_at: {
      $lte: '2020-02-23T16:00:33.036Z',
    },
  },
});

// myEntity.ts (entity definition)
interface Data {
  created_at: string;
}

@Entity({ tableName: 'mytable' })
export default class MyEntity extends BaseEntity {
  @Property({ type: 'json' })
  data: Data;
}

Generated query is:

select count(distinct("e0"."id")) as "count" from "mytable" as "e0" where "data"->'created_at'->>'$lte' = '2020-02-23T16:00:33.036Z'

$lte is misunderstood as a nested field of created_at field.

Expected behavior

select count(distinct("e0"."id")) as "count" from "mytable" as "e0" where "data"->>'created_at' >= '2020-02-23T16:00:33.036Z'

Versions

Dependency	Version
node	14.15.4
typescript	4.1.5
mikro-orm	4.4.4
pg	8.5.1