(draft) add per callback group thread isolation

[skyegalaxy]

Description

This PR adds a new prototype API to the callback group events executor, allowing for individual callback groups to be assigned a dedicated worker thread. That group's events will now be routed to a private per CBG-handle events queue instead of the shared deque, decoupling its latency from the rest of the system load.

Dedicated threads can optionally also be given a thread name, scheduling policy, priority and CPU affinity (linux only)

Example usage:

#include "rclcpp/executors/events_cbg_executor/events_cbg_executor.hpp"
using rclcpp::executors::EventsCBGExecutor;

auto node = std::make_shared<MyCoolNode>();

// Put the latency-critical work in its own mutually-exclusive callback group.
auto control_cbg =
  node->create_callback_group(rclcpp::CallbackGroupType::MutuallyExclusive);
// ...create the control timer/subscription with options.callback_group = control_cbg...

EventsCBGExecutor executor(rclcpp::ExecutorOptions(), 4);   // 4 shared pool threads

// Give the control CBG its own thread: SCHED_FIFO prio 80, pinned to CPU 3.
EventsCBGExecutor::DedicatedThreadOptions opts;
opts.name = "control";
opts.scheduling_policy = EventsCBGExecutor::DedicatedThreadOptions::SchedulingPolicy::Fifo;
opts.priority = 80;
opts.cpu_affinity = {3};
// The next line will take the above options and spawn an additional, persistent thread just for control_cbg
executor.set_dedicated_thread_for_callback_group(control_cbg, opts);  // call this BEFORE the group reaches the executor

executor.add_node(node);
executor.spin();

if your platform doesn't offer CPU pinning / affinity / scheduling policy features, callbacks can still be isolated like so, without passing in a ThreadOptions:

executor.set_dedicated_thread_for_callback_group(control_cbg);
executor.add_node(node);
executor.spin();

Fixes # (issue)

Is this user-facing behavior change?

Users will now be able to selectively isolate callback groups to their own dedicated thread, and configure the thread options at the posix level if on linux.

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Additional Information