Add festival stub entry for NixOS

[wentasah]

Festival is not available in nixpkgs. There were some attempts to package it, but it seems to be complicated. The soud_play package, which is being ported to ROS 2, depends on it, but only via exec_depend and is still useful without festival. The only place, where festival is needed is here.

Therefore, to have at least some sound_play functionality available with Nix, we add festival-stub attribute to nix-ros-overlay (the PR will be merged after this one), which allows building the relevant packages. If somebody uses the festival functionality, they'll see the error message at runtime.

[github-actions]

Thanks for sending a pull request to ROS distro!

This is an automated tool that helps check your pull request for correctness.
This tool checks a number of attributes associated with your ROS package and generates a report that helps our reviewers merge your pull request in a timely fashion. Here are a few things to consider when sending adding or updating a package to ROS Distro.
ROS Distro includes a very helpful CONTRIBUTING.md file that we recommend reading if it is your first time submitting a package.
Please also read the ROS Distro review guidelines which summarizes this release process.

ROS Distro Considerations

• ROS Distributions are created using REP-134 Standards Track as a guide.
• Your package name should comply to REP-144 ROS Package Naming
• Your package must build for all platforms and architectures on the ROS buildfarm. See REP-2000 ROS Releases and Supported Platforms for all supported platforms for your ROS Distro.
• Your package must contain an OSI approved license. Your package.xml file must also include that license in a machine readable format. See REP-149 Package Manifest Format Three Specification for additional details.
• A publicly available, open source, repository for your ROS package.
• While not required, we recommend that you create an account for ROS Discourse and subscribe to the appropriate release topic.
• If you would like, you may join our Zulip Server and ask questions in the Infrastructure General channel.

Package Considerations

Having your package included in a ROS Distro is a badge of quality, and we recommend that package developers strive to create packages of the highest quality. We recommend package developers review the following resources before submitting their package.

• REP-2004 Package Quality Declaration-- The ROS 2 TSC has created a quality rating system for ROS packages. These ratings should serve as a guide for package developers. We recommend package developers achieve a quality level of three or higher.
• Documentation -- it is recommended that ROS packages include an extensive README.md file, and API level documentation using the Sphinx documentation system.
• Maintainer Responsibilities -- the ROS 2 documentation includes a guide to ROS package maintainer responsibilities that summarizes your responsibilities as an open source maintainer. While we do not enforce these requirements on package maintainers they should be considered best practices.
• We recommend that your package should strive to conform to the ROS 2 Developer Guide and the ROS 2 Style Guide.

Need Help?

Please post your questions to Robotics Stack Exchange or refer to the Infrastructure General channel on our Zulip server.

---

For changes related to rosdep:

• ✅ Platforms for new rosdep rules are valid

For changes related to yamllint:

• ✅ All new lines of YAML pass linter checks

[mergify]

Tick the box to add this pull request to the merge queue (same as @mergifyio queue).

• Queue this pull request