Add config clone function

[jean-roland]

Closes #600

[bjsowa]

@jean-roland Would it be possible to add error checking to z_config_clone and return the int8_t result ?
Also, why is config still using _Z_OWNED_FUNCTIONS_NO_COPY_DEF and _Z_OWNED_FUNCTIONS_VALUE_NO_COPY_IMPL macros, instead of _Z_OWNED_FUNCTIONS_DEF and _Z_OWNED_FUNCTIONS_VALUE_IMPL

[jean-roland]

@bjsowa for the signature change you'll have to request it to Zenoh-C as pico is bound to follow the common C API.

I implemented a clone function and not a copy one, but I could. Anyway, I forgot to add z_config_clone to the z_clone macro so I'll have to open a new PR...