[Feature Request]: Cluster Admin HTTP APIs - part2

### Is there an existing issue for the same feature request?

- [X] I have checked the existing issues.

### Is your feature request related to a problem?

_No response_

### Describe the feature you'd like

Support following HTTP APIs:

Set node role:
```console
curl --request POST \
--url http://localhost:23820/admin/node/current \
--header 'accept: application/json' \
--header 'content-type: application/json' \
--data '
{
"role" : "leader"
} '
```

List node variables:
```console
curl --request GET \
--url http://localhost:23820/admin/variables \
--header 'accept: application/json' \
```

List node configs:
```console
curl --request GET \
--url http://localhost:23820/admin/configs \
--header 'accept: application/json' \
```

Show node variable by name:
```console
curl --request GET \
--url http://localhost:23820/admin/variables/{var_name} \
--header 'accept: application/json' \
```

### Describe implementation you've considered

_No response_

### Documentation, adoption, use case

_No response_

### Additional information

_No response_

Start by locating the HTTP API entry points for /admin/node/current, /admin/variables, and /admin/configs, then inspect how related administrative routes are registered and tested. Done means the four requested endpoints accept or return JSON as shown, including setting the node role and retrieving variables by name.

Written by the indexing model from the issue text.