Add publication cache and querying subscriber

[sashacmc]

Add publication cache and querying subscriber C bindings

For publication cache:

• ze_declare_publication_cache
• ze_undeclare_publication_cache
• ze_publication_cache_check
• ze_publication_cache_null
• ze_publication_cache_options_default

For querying subscriber:

• ze_declare_querying_subscriber
• ze_undeclare_querying_subscriber
• ze_querying_subscriber_check
• ze_querying_subscriber_null
• ze_querying_subscriber_options_default

Additionally provided bindings for required but unstable types:

• zcu_locality_t
• zcu_reply_keyexpr_t

Usage examples:

• z_pub_cache.c
• z_query_sub.c

Value of Z_CONFIG_ADD_TIMESTAMP_KEY changed to the actual one.

[Mallets]

I propose the following naming rule for everything that is unstable: zcu_xxx. Once the API is eventually stabilised, we can keep compatibility with existing code using unstable marking by adding a #define zc_xxx zcu_xxx.

[p-avital]

Close, just a couple more changes and it's ready :)