Add default implementation and builder pattern for QoS

[luca-della-vedova]

This PR adds a default implementation for QoSProfile as well as a builder pattern based on the rclcpp API.
I split it into a first commit that should be fully uncontroversial and a second commit that contains the builder API for Duration based items, which should also be OK I believe.

I intentionally left out all the methods in the other APIs that just set a profile to the requested structure, since while in C++ people might have to do something like:

const auto reliability = QoSReliabilityPolicy::Reliable;
auto profile = QoSProfile::SystemDefault().reliability(reliability);

In Rust the API is (imho) not so necessary since they can just use struct initialization, such as:

let reliability = QoSReliabilityPolicy::Reliable;
let profile = QoSProfile {
   reliability,
   ..QOS_SYSTEM_DEFAULT
};

[jhdcs]

These changes all look good to me! Only one minor comment/question about adding a comment to the default function, but I don't think it's enough to hold up the PR if we don't want to do it.

[jhdcs]

Should we make a note here that this is setting the QOS profile to the RCL default? Just to avoid any confusion?

[luca-della-vedova]

Done! 6ac6caa

[esteve]

@luca-della-vedova thanks 🙂

[esteve]

@luca-della-vedova thanks. I agree with @jhdcs about adding a note about the RCL default, could you add that? Thanks