Attach child-items to an existing work item
🛑 [-Blocked by:-] https://gitlab.com/gitlab-org/gitlab/-/issues/342752 and MVC 1 in general
## Goal
Allow the user to attach work items (child) to the current work item in view (parent).
> JTBD: When splitting prioritized initiatives or features into requirements, I want to group related slices of value and surface dependencies, so I can maximize alignment on the scope of a business goal and efficiently plan its incremental delivery.
**This object can be:**
- Child [+MVC focus+]
- Parent [-Not MVC focus-]
- Related / sibling [-Not MVC focus-]
- Blocking [-Not MVC focus-]
- Blocked by [-Not MVC focus-]
- Mentioned in [-Not MVC focus-]
**It can also be:**
- [A new object](https://gitlab.com/gitlab-org/gitlab/-/issues/337567)
- [An existing object](https://gitlab.com/gitlab-org/gitlab/-/issues/337565)
**An existing object should be:**
- Easy to search for and find for all teammates
- Easy to add without friction
- Validated as to if it makes sense to add
- Conforming with the chosen or recommended workflow
**A new object should be:**
- Easy to create
- Configured in such a way that it immediately adds value
- Validated to conform with intended workflow and rules
**New or existing objects should:**
- Give validation that they have been created or added successfully
- Give validation if they are not created or added successfully
- Configured within the context of the current object
- Clearly demonstrate relationship to current object
### Flow ideation

## Proposal
[+To be added+]
epic