class-transformer feat: extends discriminator lookup

Description

I truly appreciate the possibility to have polymorphic objects in my models. Unfortunately, it's not always possible to add a new specific property to make the discriminator lookup working and moreover, quite often the discriminator itself can already be found in the parent object (in my use cases at least).

My proposal is the following :

add a parentProperty flag in DiscriminatorDescriptor to indicate if we should look for the discriminator in the parent
allow DiscriminatorDescriptor.subTypes.name to be string or a number
allow DiscriminatorDescriptor.propertyto be either string or string[] to provide more complex path
create new helpers (getDiscriminator, setFromDiscriminator, removeDiscriminator) in TransformOperationExecutor.ts to delegate lookup and object updates.
update transform flow when the discriminator options are provided

Checklist

[x] the pull request title describes what this PR does (not a vague title like Update index.md)
[x] the pull request targets the default branch of the repository (develop)
[x] the code follows the established code style of the repository
- npm run prettier:check passes
- npm run lint:check passes
[x] tests are added for the changes I made (if any source code was modified)
[x] documentation added or updated
[x] I have run the project locally and verified that there are no errors
[x] add extra unit tests in custom-transform.spec