Add project: opcua-client-gui

Nobody has claimed this yet.

## Basic info

**Application name**: opcua-client-gui
**Application repo link**: https://github.com/FreeOpcUa/opcua-client-gui
**Application home link**: https://freeopcua.github.io
**Application description**: Cross-platform GUI client for the OPC-UA protocol used by programmable logic controllers and other industrial automation technology (`linux`, `windows`, `mac`, `pyqt`)

## Qualifications

There are a ton of great Python projects out there, but what makes an
application "Awesome™"? Please check that all of the following criteria apply:

- [x] Free software with an online source repository.
- [x] Using Python for a considerable part of their functionality.
- [x] Well-known, or at least prominently used in an identifiable niche.
- [x] Maintained or otherwise demonstrably still functional on relevant platforms.
- [x] An application, not a library or framework.

Installable via `pip install opcua-client` ([PyPI page](https://pypi.org/project/opcua-client/))

## Additional notability info

It's the canonical debugging tool for the underlying `python-opcua` and `asyncua` libraries which have a combined ~650 Github stars and ~300 forks. For the small niche of Python usage in automation, this is substantial.

The project is mentioned on the [OPC-UA standard's Wikipedia page](https://en.wikipedia.org/wiki/OPC_Unified_Architecture#Python_implementation).

Review the repository's application list and nearby entries, then add opcua-client-gui in the established format using the supplied repository link, home link, description, and qualification details. Done means the project appears as a complete, consistent entry in the list.

Written by the indexing model from the issue text.