Enable zenoh/multilink feature

[Hugal31]

Description

Enable the zenoh/transport_multilink feature.

Is this user-facing behavior change?

Users will now be able to connect two nodes with more than one link. User not using this feature will not see any change.

Did you use Generative AI?

No.

Additional Information

I have seen multiple examples hinting the use of multilink (e.g. the doc mentioning transport being chosen depending on the reliability and this example for an issue), yet, it was not enable.

[Yadunund]

Thanks for the pull request.

We're thinking about how to best support more users who want to exploit capabilities of Zenoh in rmw_zenoh. Having PRs that add features one by one may not make the most sense.

We could have enabled all features by default but chose to selectively enable those that we strictly need to minimize the impact on the ROS Buildfarm as building zenoh_cpp_vendor is a time and memory consuming step. We're already seeing build and testing jobs fail due to timeouts from the increased buildtime. We've also seen complaints on excessive disk usage during builds #447.

One long-term strategy in mind is to not distribute zenoh-cpp via the Buildfarm and instead install zenoh-cpp debs distributed directly from the Official Ubuntu Repository or the Eclipse one. These debs will have all features enabled.

To address the concerns with increased memory and build time on the Buildfarm, it would help if you could benchmark the increase with all features enabled (instead of just the one in this PR). We can make a decision based on that.

In the meanwhile, our recommendation is to build rmw_zenoh from source by overwriting the ZENOHC_CARGO_FLAGS as needed.