DeleteOperation(T) deletes_dependent feature request

Nobody has claimed this yet.

My recurrent pattern is that my `DeleteOperations` look like this:
```crystal
class DeleteCompany < Avram::DeleteOperation(Company)
before_delete :do_before_delete

def do_before_delete
delete_dependent(record.page!, record.posts!)
end
end
```
...where `page` is a `has_one` relationship and `post` is a `has_many`. `delete_dependent` calls the delete operation on every object passed in its arguments, whether they are singular or arrays. This could be done in a macro, for example:
```crystal
class DeleteCompany < Avram::DeleteOperation(Company)
deletes_dependent :page, :posts
end
```
It's a little easier in my app because my models provide a `#delete_operation(**named_attributes)` instance method that returns a new delete operation for that record. There would have to be a naming convention for the delete operation for this to work.

Start at DeleteOperation(T) and the existing delete_dependent usage shown in the issue; inspect how dependent records and delete operations are represented. Resolve the naming convention required by a deletes_dependent macro for both has_one and has_many relationships. Done means the macro handles singular and collection dependents, with tests covering each case.

Written by the indexing model from the issue text.