z_open should accept z_open_options_t to allow for future extensions

[DenisBiryukov91]

Describe the release item

z_open signature should be changed to
z_error_t z_open(struct z_owned_session_t *this_, struct z_owned_config_t *config, const z_open_options_t *options);

This will allow providing more options in the future without breaking the api.

[milyin]

There was also a request from @kydos to provide open function without required config parameter, as usually our config is default: eclipse-zenoh/zenoh#805, This is not decided yet and may be it makes sense to think about these issues together.

On the zenoh-c side we can put pointer to config inside options structure, allow it to be nullptr and ,provide z_open_options_default with null pointer to config. Or better name it z_session_options_t ?

The code will look like:

z_owned_session_t session;
int err = z_open(&session, z_open_options_default());

Here I assume that we passing options by value, which is not the case at this moment. Issue #401 was created for this

[milyin]

z_owned_config_t config;
z_config_new(&config);
z_owned_session_t session;
z_open_options_t opts;
z_open_options_default(&opts);
opts.config = z_move(config);
int err = z_open(&session, &opts);

vs

z_owned_config_t config;
z_config_new(&config);
z_owned_session_t session;
z_open_options_t opts = z_open_options_default();
opts.config = z_move(config);
int err = z_open(&session, opts);

[DenisBiryukov91]

To allow providing options inline and specifying only non-default values we can probably try to use macros as proposed here https://stackoverflow.com/questions/13716913/default-value-for-struct-member-in-c.