Mark functions as [[nodiscard]]

[tambry]

Today I wrote code analogous to this:

auto config{zenoh::Config::create_default()};
config.from_str("...");

Unfortunately turns out that the second call is completely useless!
If it had been marked [[nodiscard]] it would've saved me a few hours of debugging.
And so should probably a majority of functions, which only return values and don't have other side effects.