First step towards versioning the message definitions indepedently.

Makes the distinction between raw data and data messages clearer.

Our upcoming ros2<->dora bridge will allow nodes to subscribe to ROS2 topics, which results in a second stream of events. By providing a merge function, this ROS2 event stream can be merged together with the dora event stream.

Instead of doing an additional copy to send them from the operator thread to the runtime thread.

This commit uses the new `allocate_data_sample` and `send_output_sample` methods introduced in d7cd370.

We currently need an additional request to prepare zero-copy shared memory messages. For small messages, it might be faster to avoid this extra round-trip and send the message directly over TCP. This commit implements support for this. Messages smaller than a threshold (currently set to 4096 bytes) are sent via TCP, while larger messages still use shared memory.

This step also enables future optimizations such as queueing output messages in order to improve the throughput.

It is not guaranteed that the `EventStream` is dropped before the `DoraNode`. If it is dropped later on the same thread, this `join` leads to a deadlock.

We cannot wait until all operators of the runtime node are finished because operators might be subscribed to other operators.

To allow for individual testing and benchmarking. This will also enable reuse of the library in other projects.

The goal is to avoid the latency of the TCP socket.

On Windows, the size of the shared memory region is rounded up to the next multiple of the page size. By slicing the region to the expected size we allow nodes to assume that they get exactly the requsted size.

Useful in case of emergencies.

Changes the message format from raw data bytes to a higher-level `Message` struct serialized with capnproto. In addition to the raw data, which is sent as a byte array as before, the `Message` struct features `metadata` field. This metadata field can be used to pass open telemetry contexts, deadlines, etc.

To make it usable for other projects.