Add parameter callbacks

[azerupi]

Adds three new capabilities to the parameter API:

1. ParameterBuilder::validate() to register a callback that can reject parameter changes. Called during declaration (for the initial value) and on every subsequent set, both programmatic and via service calls. Runs after range checks, before the value is written.
2. MandatoryParameter::on_change() / OptionalParameter::on_change() to register a callback invoked after a parameter value is successfully changed. Intended for side effects (reconfiguring algorithms, logging, etc.). Callbacks are invoked outside the parameter map lock to avoid deadlocks.
3. MandatoryParameter::subscribe() / OptionalParameter::subscribe() that return a ParameterSubscription for async notification of changes via tokio::sync::watch. Supports changed(), get(), and wait_for(predicate). The naming might be slightly confusing with subscribers already being a foundational ROS 2 concept, let me know if there are better naming ideas.

Also adds ValidationFailed and InitialValueRejected error variants to ParameterValueError and DeclarationError respectively.

Examples can be found here: https://github.com/azerupi/ros2_rust_examples/tree/parameter_callback_v2

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Disclaimer AI was used to help write this PR