Vendor rcl_interfaces as a private module into rclrs

[nnmm]

The parameter services require the rcl_interfaces message and service types. I think this option is preferable to publishing rcl_interfaces on crates.io.

The script included in this PR produces the vendor module inside rclrs by copying the generated code for the rcl_interfaces package and its dependency builtin_interfaces and adjusting the submodule paths in the code.
If these packages, or the rosidl_generator_rs package, get changed, we can update the vendor module by running this script.

[esteve]

LGTM, but I'll leave it to @jhdcs to approve it as I see that CI is failing.

@nnmm can you update the description of this PR and add a docstring to the Python script with a description of what it does and how to use it? Thanks

For a future PR, or maybe even this one, I'd add the test_interfaces messages so that we can integrate the tests here, I prefer tests to be in the same crate/package, they are easier to maintain and also serve as examples of how to use the API

[nnmm]

@esteve I had planned to just create an alias ExampleMessage for one of the rcl_interfaces messages that we can use in doctests. For most tests, the contents of the message being used don't matter. So I propose to do that for now, and if/when we see a need for test_msgs specifically, we can consider vendoring that too.

[jhdcs]

This looks fine to me as well, though I don't see a failing CI. Was that a previous commit?

[nnmm]

@jhdcs Yes, that was a previous commit. I've also added the comment requested by @esteve.

[nnmm]

@jhdcs Could you reapprove this? I've added a commit to add these dependencies to package.xml, since we will still depend on the C libraries from these packages.

[nnmm]

Thanks!