Add ROS2 support for calling server-advertised services

[achim-k]

Public-Facing Changes

• Add ROS2 support for calling server-advertised services

Description
Based on #136

• Adds experimental ROS2 support for advertising/unadvertising services and allowing clients to call them
  • Implements the services spec that was added in Extend spec with services capability ws-protocol#344

Implementation details:

The main problem is, that symbols from getting the service type support are missing from the rosidl_typesupport_cpp libraries that are generated for each msg/srv package (see ros2/rosidl_typesupport#122). There is an open pull request (ros2/rosidl_typesupport#114) to fix that, but so far it hasn't been merged.

I found a working, yet little bit hacky, workaround for this problem which is described more in detail here:

ros-foxglove-bridge/ros2_foxglove_bridge/src/generic_client.cpp

Lines 59 to 89 in f978c18

	/**
	* The default symbol names for getting type support handles for services are missing from the
	* rosidl_typesupport_cpp shared libraries, see
	* https://github.com/ros2/rosidl_typesupport/issues/122
	*
	* We can however, as a (hacky) workaround, use other symbols defined in the shared library.
	* With `nm -C -D /opt/ros/humble/lib/libtest_msgs__rosidl_typesupport_cpp.so` we see that there is
	* `rosidl_service_type_support_t const*
	* rosidl_typesupport_cpp::get_service_type_support_handle<test_msgs::srv::BasicTypes>()` which
	* mangled becomes
	* `_ZN22rosidl_typesupport_cpp31get_service_type_support_handleIN9test_msgs3srv10BasicTypesEEEPK29rosidl_service_type_support_tv`
	* This is the same for galactic, humble and rolling (tested with gcc / clang)
	*
	* This function produces the mangled symbol name for a given service type.
	*
	* \param[in] serviceType The service type, e.g. "test_msgs/srv/BasicTypes"
	* \return Symbol name for getting the service type support handle
	*/
	std::string getServiceTypeSupportHandleSymbolName(const std::string& serviceType) {
	const auto [pkgName, middleModule, typeName] = extract_type_identifier(serviceType);
	const auto lengthPrefixedString = [](const std::string& s) {
	return std::to_string(s.size()) + s;
	};
	return "_ZN" + lengthPrefixedString(TYPESUPPORT_LIB_NAME) +
	lengthPrefixedString("get_service_type_support_handle") + "IN" +
	lengthPrefixedString(pkgName) +
	lengthPrefixedString(middleModule.empty() ? "srv" : middleModule) +
	lengthPrefixedString(typeName) + "EEEPK" +
	lengthPrefixedString("rosidl_service_type_support_t") + "v";
	}

All in all, the implementation is working, but I would consider it as experimental for now. I am not sure if this approach would work on windows / mac