fix: adminspace encodings

[gabrik]

After the removal of Value the encoding of the Admin Space have been lost.

So for instance this:

{
        "key": "@/router/8f2aef5a89c14100b70184094f5bdea9/status/plugins/storage_manager/volumes/file-system-volume",
        "value": {"root":"/home/vscode/.zenoh/zenoh_backend_fs","version":"0.11.0-dev"},
        "encoding": "application/json",
        "time": null
    }

becomes this:

{
        "key": "@/router/8f2aef5a89c14100b70184094f5bdea9/status/plugins/storage_manager/volumes/file-system-volume",
        "value": "eyJyb290IjoiL2hvbWUvdnNjb2RlLy56ZW5vaC96ZW5vaF9iYWNrZW5kX2ZzIiwidmVyc2lvbiI6IjAuMTEuMC1kZXYifQ==",
        "encoding": "zenoh/bytes",
        "time": null
    }

This PR aims at solving this bug.

[gabrik]

There are still some Key that provide a base64 response:

• [x] @/router/<id>/status/<plugin>/__path__
• [x] @/router/<id>/linkstate/routers
• [x] @/router/<id>/metrics

still investigating why

[gabrik]

So a z_get gets them correct, the REST API does not...

[gabrik]

Updated the Encoding matching in the REST plugin.