[Shadergraph] check for description attribute in pluginfo to assign a description to a node port

[QuackCola]

Summary

A small tweak so that PlugInfo checks for the DescriptionAttribute

Motivation & Context

Lets users define descriptions for NodeInput and NodeResult properties of a node class so that when
hovering over the respective ports on the node the assigned description is shown.

Implementation Details

Just a simple attribute check in the PlugInfo class contructor.

Screenshots / Videos (if applicable)

In-Graph test :

Example Definition in a node class :

Checklist

• Code follows existing style and conventions
• No unnecessary formatting or unrelated changes
• Public APIs are documented (if applicable)
• Unit tests added where applicable and all passing
• I’m okay with this PR being rejected or requested to change 🙂