Honor optional field in Angular migrations

[yjaaidi]

Since Angular 18, Angular has a new concept of optional migrations (e.g. @angular/core's provide-initializer or @angular/cli application builder).

When the optional field is set to true, the user is prompted to run that migration. Users can also decide to run the migration later on using commands such as ng update @angular/core --migrate-only --from 18 --to 19.

It would be nice if:

• Nx would honor the optional field and prompt the user
• Nx would provide an option to re-run past optional migrations