feat: add serialized pub/sub APIs

[bresilla]

What

• Re-export rcl_bindings (pub mod rcl_bindings) to enable low-level extensions.
• Add first-class serialized messaging types:
  • SerializedMessage
  • SerializedSubscription
  • SerializedPublisher
• Add Node::create_serialized_subscription(...) and Node::create_serialized_publisher(...).

Why

Enables native tools (e.g. rosbag MCAP record/play) to work with raw serialized CDR bytes without needing generated Rust message crates.

Notes

• Uses existing dynamic message typesupport resolution (DynamicMessageMetadata) so it works for any installed message type.
• Does not change existing typed or dynamic pub/sub behavior.

[Copilot]

Pull request overview

Re-exports the generated low-level rcl_bindings module from rclrs so downstream crates can use raw rcl/rmw APIs without duplicating bindings.

Changes:

• Makes the internal rcl_bindings module public (pub mod rcl_bindings;).

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[Copilot]

lib.rs has #![warn(missing_docs)], and CI runs cargo rustdoc -- -D warnings / cargo clippy ... -D warnings. Making rcl_bindings public without a doc comment will emit a missing_docs warning for this module item and fail CI. Add a /// doc comment for pub mod rcl_bindings; (or annotate this item with #[allow(missing_docs)] / #[doc(hidden)] if you don't want it documented).

Suggested change

	/// Low-level bindings to the underlying ROS 2 `rcl` C API.

[mxgrey]

Is this possibly a duplicate of the feature introduced by #492 ?