Add curve functions wrappers

[cacharle]

Fixes #670

[cacharle]

Not sure why the CI is failing

[gummif]

Could you try bumping the C++ version of the gcc-9 test to gcc-11?

[cacharle]

I think I addressed all of your comments, the CI is still failing tho 🤔

[gummif]

Ok looks good. Just need to figure out this CI issue.

[cacharle]

Other PRs have failing CI aswell, probably some dependency wasn't fixed

[cacharle]

Need to build libzmq with ENABLE_CURVE (in matrix?), fix the lcov error, add libsodium and clean the git history from all the CI commits

[cacharle]

Only failure left is the coverage where it seems there is different versions for my test file:

Processing unit_tests.dir/curve.cpp.gcda
/home/runner/work/cppzmq/cppzmq/build/tests/CMakeFiles/unit_tests.dir/curve.cpp.gcno:version 'B23*', prefer 'B14*'
geninfo: ERROR: GCOV failed for /home/runner/work/cppzmq/cppzmq/build/tests/CMakeFiles/unit_tests.dir/curve.cpp.gcda!

[cacharle]

Okay, let me know if you're happy with the end state of the CI (I changed to gcc11, removed some warnings when building libzmq and added a job for the curve feature)
I'll squash all the commits after

[coveralls]

Pull Request Test Coverage Report for Build 20334274751

Details

• 13 of 18 (72.22%) changed or added relevant lines in 1 file are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.3%) to 88.057%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
zmq.hpp	13	18	72.22%

Files with Coverage Reduction	New Missed Lines	%
zmq.hpp	1	85.71%
zmq_addon.hpp	5	94.18%

Totals
Change from base Build 17909983277:	-0.3%
Covered Lines:	929
Relevant Lines:	1055

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 20335143482

Details

• 13 of 18 (72.22%) changed or added relevant lines in 1 file are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.3%) to 88.057%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
zmq.hpp	13	18	72.22%

Files with Coverage Reduction	New Missed Lines	%
zmq.hpp	1	85.71%
zmq_addon.hpp	5	94.18%

Totals
Change from base Build 17909983277:	-0.3%
Covered Lines:	929
Relevant Lines:	1055

---

💛 - Coveralls

[gummif]

Nice, thanks.