Start creating a Python ROS2 bridge

[phil-opp]

• Make compile-time ROS2 message generation optional
  • We will ship a prebuilt version of the dora-ros2-bridge crate via pip, so we don't want to generate the ROS2 message structs at build time.
  • Instead, we will create the message structs at runtime, using the separate msg-gen crate that was created in Split parsing into separate crate #1.
• Create a new dora-ros2-bridge-python crate that uses maturin and pyo3 to be usable from Python.
  • The crate depends on dora-ros2-bridge without default features, i.e. without compile-time ROS2 message generation.
• Provide Python-compatible wrapper types for creating nodes, publishers, subscribers, and QoS settings.
• Experiment with dynamically-typed messages using the pythonize crate.
  • Unfortunately, this doesn't seem to work. The ROS2 CDR serialization format does not encode type information, so we have to know the target type in order to deserialize a message.
  • For publishing messages, it seems like invalid messages are silently ignored, which makes debugging difficult. So for static message types are preferable for publishing too.

[phil-opp]

I moved out the ros2-client and rustdds changes into separate crates. The plan is to upstream some or all of these updates. Relevant PRs:

• Fix overly strict lifetime requirements for deserialization helpers Atostek/RustDDS#291
• Add DeserializeSeed support RustDDS#2
• Add support for subscriptions that use DeserializeSeed ros2-client#1

[phil-opp]

Given that it might take some time until the upstream changes are merged, I don't think that we should wait for it. The dora-ros2-bridge is still experimental anyway, so having some temporary git dependencies in it would be fine with me. What do you think @haixuanTao ?

[haixuanTao]

I see. Sure. Having git dependenccy in the Cargo.toml is totally fine.

[phil-opp]

Ok great! Do you want to do a quick review or are you fine with merging this as-is?

[haixuanTao]

I'm okay with merging this as is, as there is not a lot history to begin with.

I'll test maybe later if that's ok!