[dmeoli]

At the moment cmake --install does nothing, so a project that wants to link Clarabel.cpp without adding it as a subdirectory has to copy the headers and rust_wrapper/target/release/libclarabel_c.* by hand and write its own find module. I use Clarabel.cpp as a QP backend of a C++ library (SMS++) and of its packages, where this is what I ended up doing.

This PR adds:

• install() rules for the headers (include/ as it is), the shared and the static library, with GNUInstallDirs, and on Windows the DLL in bin;
• a package lib/cmake/Clarabel/ClarabelConfig.cmake (plus the version file) that defines the imported targets Clarabel::clarabel_c and Clarabel::clarabel_c_static, with the include directory, the FEATURE_* definitions of the enabled cargo features, and Eigen3::Eigen if Eigen is found (the C++ headers need it, the C ones do not);
• the same two names as aliases of libclarabel_c_shared and libclarabel_c_static, so that a consumer is written the same way with find_package and with add_subdirectory;
• the option CLARABEL_BUILD_EXAMPLES (default ON, as now); since the examples were the only targets depending on the cargo target, libclarabel_c is now built by ALL, otherwise with the examples off nothing would trigger cargo;
• a few lines on installation in the README.

Two details I ran into while testing:

• the static library needs the system libraries of the Rust standard library (cargo rustc -- --print native-static-libs gives -lgcc_s -lutil -lrt -lpthread -lm -ldl -lc here). Without them, linking an executable to the current libclarabel_c_static fails with undefined references to log, exp and so on, so on Linux both libclarabel_c_static and the installed static target now link Threads::Threads ${CMAKE_DL_LIBS} m rt util. I did not add a list for macOS and Windows, which I cannot test;
• libclarabel_c.so has no SONAME, so linking it by full path records that absolute path in the executable; the installed target is marked IMPORTED_NO_SONAME, and the executable records libclarabel_c.so, found through its RUNPATH.

I tested it on Linux (Ubuntu 24.04, GCC 13.3, CMake 3.28, Rust stable) with a separate project built against the installed tree, which compiles examples/c/example_qp.c against both targets and examples/cpp/example_qp.cpp against the shared one (all three solve), and with the same C example linked to Clarabel::clarabel_c_static through add_subdirectory. The default build with the examples is unchanged, and -DCLARABEL_CARGO_FEATURES=serde,faer-sparse gives FEATURE_FAER_SPARSE;FEATURE_SERDE in the installed package.

#72 also adds install rules, for the use in acados; this PR installs the headers and a package with targets as well, and keeps the build directory logic as it is.