Add mrs_lib::ServiceClient

[pum1k]

Summary

Add updated mrs_lib::ServiceClient, a rclcpp::Client wrapper with coroutines support.

This class replaces the mrs_lib::ServiceClientHanlder. Main changes:

• construction from node interfaces (does not need node shared ptr)
• no default constructor - prevents invalid states by requiring initialization
• better support for coroutines
• error messages with std::expected (instead of previous std::optional)