add querier (#837)

* add querier support and corresponding examples;
align z_queruyable, z_get and z_pub examples with rust;

* fix z_get example

* docs update

* remove non-portable strcasecmp

* docs fix

* cargo lock update

* add zc_undeclare_matching_listener

* add --complete option to z_queryable_with_channels examples

* move querier under unstable

* switch branch to zenoh/main

* docs update

* fix branch in cargo.toml.in