Add Extend and FromIterator impl for rosidl_runtime_rs::{String, WString}

[nnmm]

It would be nice if the string types in rosidl_runtime_rs could be built/extended from iterators, like the regular string type.

For this, we'd need implementations for Extend<char> and FromIterator<char> for both types, at least.

[maspe36]

An important thing to note, is that this feature gives the string types interior mutability. So these string types would have to lose the Sync impl.

https://github.com/ros2-rust/ros2_rust/blob/main/rosidl_runtime_rs/src/string.rs#L230

[nnmm]

I think we wouldn't need to lose the Sync marker, since the Extend trait takes &mut self and not &self.

[maspe36]

Ahh, gotcha. Yeah even rust's String is Sync
https://doc.rust-lang.org/std/string/struct.String.html#impl-Sync-for-String

Updated in the PR

[maspe36]

I believe this issue should be closed now that #293 has been merged