[Feature Request]: Subgraph Visual Seperators/Headers

Allow for subgraphs to have seperators/headers so that inputs can be categorized/organized for better readability.

This allows users to label and organize their subgraph node inputs. And allows us to communicate to other users what inputs do what and how they relate to each other

### Proposed workflow

When inside a subgraph:
- Right-click on sugraph input node
- Menu:Add Header
A new dummy input socket is created to be used as a header, the socket name is the external header

### Additional information

A Blender "group node" example:

- Related inputs are grouped under the same user defined headers
- Very good readability ✓
- Very good for communicating to other users what the functionality of inputs are for, how they relate to each other ✓

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-4653-Feature-Request-Subgraph-Visual-Seperators-Headers-2436d73d3650819582dde53b99e9717e) by [Unito](https://www.unito.io)