Config API alignment

[DenisBiryukov91]

Describe the release item

Rust exposes public api for native config entries access / modification via native zenoh_config::Config struct. Should we provide similar native access to config entries in zenoh-c? As of today we can only load config from json, or add/modify config entries by passing json key-value pairs.

We should likely also expose Config::from_env method to allow loading config from path stored in env variable.