Install Zenoh under `include/zenoh`

[doganulus]

Describe the feature

A modern best practice for installing C/C++ libraries is to place their headers within a dedicated namespace directory under include. Currently, zenoh-c installs its headers directly into ${INSTALL_PREFIX}/include, which can become cluttered as more libraries are installed.

Hence, this issue proposes a change to install Zenoh C/C++ headers under ${INSTALL_PREFIX}/include/zenoh to provide a cleaner, namespaced layout.

Note that this is a breaking change: examples and downstream projects will need to update their include paths if this is adopted. Additionally, synced changes need to be applied to zenoh-pico and zenoh-cpp as well.

Some open issues on the topic:

• Reorganize the include files zenoh-pico#80

[milyin]

I agree with this approach, but for now API is fixed and is not going to be refactored until next major library version. So this should be postponed until zenoh 2.0.

[doganulus]

Maybe a gradual approach would be better to redirect existing headers to their new locations as follows:

// ${INSTALL_PREFIX}/include/zenoh.h

#warning "This header is deprecated and will be removed in a future release. Please use `#include <zenoh/zenoh.h>` instead."
#include "zenoh/zenoh.h"

This would give you time to update the docs and examples as well.

[milyin]

This is still kind of breaking change. If existing approach doesn't impose serious issues right now, I'd keep things as is at this moment.

[doganulus]

Sure. It just feels inconsistent when installing other libraries alongside Zenoh, since each of them uses its own dedicated namespace.