[Feature] Handle zenoh keyexpr with std::string_view rather than const std::string &

[YuanYuYuan]

The zenoh key expression is not null-terminated. That's why we need to attain the pointer with its length while printing the string in zenoh-c.

printf("%.*s", (int)z_string_len(z_loan(keyexpr)), z_string_data(z_loan(keyexpr));

Currently, we use const std::string & to process the passing keyexpr after conversion. This forces us to copy the context by creating a std::string. The most efficient way to borrow and read the zenoh keyexpr is

1. Use z_keyexpr_as_view_string to read the z_view_string_t from a loaned keyexpr.
2. Pass the zenoh view string by calling std::string_view(z_string_data, z_string_len).

NOTE
We need to mind the lifetime of owned key expr. Ideally, all the rmw_zenoh entities should own the data rather than a reference to somewhere.

[YuanYuYuan]

A relevant discussion before on RCLCPP. ros2/rclcpp#2634

[clalancette]

As I mentioned in ros2/rclcpp#2634 (comment), I'm not a huge fan of std::string_view; it just seems incredibly easy to get your lifetimes wrong, and thus to access already-freed memory. Passing a constref around is equivalent to passing a pointer, i.e. extremely cheap. So I'm not convinced that switching to a std::string_view will be more efficient, and it is a loaded footgun just waiting for someone to pull the trigger.

In other words, I'm not in favor of this unless it is shown that the processing of our keyexpression is causing a huge slowdown.