Use project version in default User-Agent

[dkropachev]

Summary

• publish the CMake project version as a public compile definition for consumers
• use that version in the default Config::user_agent, with a devel fallback when no version definition is available
• add config tests for the versioned default and empty-string override

Closes #52

Tests

• make check
• make test-unit
• cmake --install build --prefix build/install && rg -n "SCYLLADB_ALTERNATOR_CLIENT_CPP_VERSION|INTERFACE_COMPILE_DEFINITIONS" build/install/lib/cmake/scylladb-alternator-client-cpp