Add Debian packaging

[Daniel-Fundarek]

This pull request adds Debian package generation via CPack. The generated package declares libzmq3-dev as a dependency, so developers only need to install the cppzmq package to obtain full cppzmq development support.

[Copilot]

Pull request overview

This PR introduces Debian package generation for cppzmq via CPack and updates the build documentation to describe an apt-based installation flow.

Changes:

• Add CPack configuration in CMakeLists.txt to generate a DEB and declare a libzmq3-dev dependency.
• Update README.md build steps to describe building and installing the generated Debian package.
• Change the default for CPPZMQ_BUILD_TESTS from ON to OFF.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
README.md	Adds Debian/apt-based build + install steps for the new CPack DEB output.
CMakeLists.txt	Adds DEB packaging configuration via CPack and changes the default test-build option.

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[gummif]

This is quite specific to debian based systems with libzmq3-dev as system dependency. Not sure this is the right place for this.

[Daniel-Fundarek]

Yes, it is indeed targeted to Debian only build, other builds can still build this project from source or someone can add packing for other builds. This is what I was missing when I wanted to build apps with zmq and CPP bindings. This modification allows users to easily provide build dependency packaged into Debian, without needing to build cppzmq and libzmq on each device. Do you think this change is beneficial for this project, if so where should this change be implemented?