feat(rclrs): add Clock sleep and async sleep APIs

[PvRao-29]

Summary

• Adds Clock::sleep_for, sleep_until, async_sleep_for, and async_sleep_until, aligned with rclcpp semantics
• Adds ros_time_is_active() helper used by the sleep loop
• Async sleep yields to the runtime so it can be used under use_sim_time without blocking the executor
  Fixes Add support for Clock sleep_for/sleep_until #604

Design notes

• Poll-loop implementation (10ms tick) rather than jump callbacks; keeps the diff small and avoids overlap with Feature request: Clock Jump Callbacks #524
• Sync sleep_for can still deadlock under sim time in a single-threaded executor callback; documented in rustdoc, with async variants as the recommended path
• Returns bool like rclcpp (true = target reached, false = context invalid)

Test plan

• cargo fmt
• cargo clippy --all-targets (pre-existing warnings only)
• cargo test clock — 13 passed including 4 new clock sleep tests
• CI (rust-stable / rust-minimal)