Cannot use external kernal via allow_external_kernels and kernels.external_connection_dir

Description

And external program is serving a kernel (in my case using the xeus code) and jupyverse is started with the options

 --set "kernels.external_connection_dir=<directory_containing_connection.json> --set "kernels.allow_external_kernels=true"

In the jupyverse webserver the kernel is not visible

Note: it maybe that that I'm misunderstanding the intention of the _kernels.allow_externalkernels and _kernels.external_connectiondir options. This has also been discussed on the Jupyter Discourse Forym

AFAICT there is code to pickup and existing connection.json file but this does not configure a connectable kernel in the web interface.

Reproduce

Start an external kernel with a connection.json file e.g. :

{
"transport": "tcp",
    "ip": "127.0.0.1",
"control_port": 55684,
"shell_port":  60315,
"stdin_port":  55844,
"iopub_port":  52554,
"hb_port":  63106,
"signature_scheme": "hmac-sha256",
"key": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
"kernel_name": "MyKernel"
}

Start jupyverse pointing to the directory containing this kernel

jupyverse.exe" --set "kernels.external_connection_dir=<connection.json_dir>" --set "kernels.allow_external_kernels=true" --port=8800

Expected behavior

Kernel MyKernel will be an option in the web interface

Context

Operating System and version: Windows 11
Browser and version: Firefox 122.0 (64-bit)
Jupyter Server version: 0.4.2

I added some logger.info statements to check that the connection.json file was being read - see the ouput lines with ** below

Command Line Output

```
[2024-02-06 09:20:13,172 INFO] Running in development mode
[2024-02-06 09:20:13,368 INFO] Starting application
[2024-02-06 09:20:18,112 INFO] Scanning kernel config in D:\TempProj\DevBundle\Jupyter\install\Debug\external_kernels\ManiVault **
[2024-02-06 09:20:18,147 INFO] Processing connection files
[2024-02-06 09:20:18,147 INFO] Processing file at path D:\TempProj\DevBundle\Jupyter\install\Debug\external_kernels\ManiVault\connection.json **
[2024-02-06 09:20:18,148 INFO] Adding kernel : MyKernel **
[2024-02-06 09:20:18,269 INFO]
[2024-02-06 09:20:18,269 INFO] To access the server, copy and paste this URL:
[2024-02-06 09:20:18,269 INFO] http://127.0.0.1:8800/?token=5c56ca0a7a474549afc3fcc3b88f6944
[2024-02-06 09:20:18,269 INFO]
[2024-02-06 09:20:18,380 INFO] Started server process [49084]
[2024-02-06 09:20:18,380 INFO] Uvicorn running on http://127.0.0.1:8800 (Press CTRL+C to quit)
```

Browser Output

MyKernel is not visible in the select kernel options

jupyter-server / jupyverse