Expose ServiceType in Service public API

[SayHey]

Description

The PR exposes ServiceType in rclcpp::Service public API.

Is this user-facing behavior change?

Yes, the change unlocks deduction of the ServiceType of an already declared rclcpp::Service instance, thus avoiding duplicating type information.
Here is an example:

auto init_service = []<typename T>(T& service, const std::string& name, const auto& cb) {
  service = create_service<typename T::element_type::ServiceType>(name, cb);
};

Usage:

rclcpp::Service<long_namespace::MyServiceType>::SharedPtr my_service_; // usually a class member
...
init_service(my_service_, "my_service", my_service_callback); // usually happens in constructor
// my_service_ = create_service<long_namespace::MyServiceType>("my_service", my_service_callback); // current practice

Did you use Generative AI?

No

[jmachowinski]

I'm good with the change, but want a second opinion on the naming.
E.g. ServiceType or ServiceMsgType etc.

@fujitatomoya @mjcarroll any objections or should we just ship it ?

[fujitatomoya]

i am good to go with it.

[fujitatomoya]

Pulls: #3088
Gist: https://gist.githubusercontent.com/fujitatomoya/0ca97ee0dff9c5464ae8e243ce4bd8d4/raw/12c4388ad5eb92e7ea4dfaa2884c6e8b1fe86b30/ros2.repos
BUILD args: --packages-above-and-dependencies rclcpp
TEST args: --packages-above rclcpp
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/18362

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