Add optional __path__ and __required__ fields in Config deserialization

[fuzzypixelz]

Fixes #19.

The issue stems from Config denying unknown fields such as __path__ and __required__. As this data is (currently) unused by Web Server, 'deserializing' it into serde::de::IgnoredAny is the most straightforward solution.

Note that other Zenoh plugins such as zenoh-plugin-dds do deserialize these fields, but IIUC they do not use them afterwards. If true, I will open a PR to mark them as IgnoredAny in all plugins.

[JEnoch]

Thanks for addressing this forgotten issue!

I think that even if not used yet, those fields can be useful, and at least displayed in the admin space.
They shall be consistent with the description made in DEFAULT_CONFIG.json5 and with the code reading them in commons/zenoh-config/src/lib.rs. Thus, they shall be defined as such:

    __path__: Option<bool>,
    __required__: Option<Vec<String>>,

And I see their definitions are not consistent across plugins. See:

• REST plugin
• DDS plugin
• ROS 2 plugin
• MQTT plugin

Could you please open PR for those ?

[JEnoch]

LGTM!