A child project cannot have more than one parent

### Current Behavior

A child project cannot have more than one parent. This seems to be a strong limitation.
In fact when you create a new version of a collecting project, some the children projects are the same of the previous version.
I think for this reason, cloning a parent project does not clone also children projects.
Unluckily DependecyTrack cannot handle this situation.
It would be very useful if one could reuse an existing version o a project within more than one summarizing projects

### Proposed Behavior

A child project should be able to be linked to more than one parent project

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this enhancement was already requested