[Build] Document USE_CXL and ENABLE_MULTI_PROTOCOL as CMake options

[stmatengss]

Problem

USE_CXL and ENABLE_MULTI_PROTOCOL were undocumented compile-time flags only discoverable by reading #ifdef guards in source code. They had no option() declarations, no help strings, and no add_compile_definitions() wiring in CMake. Users had to pass them as opaque -D flags with no guidance.

Fix

Add option() declarations with descriptive help strings and add_compile_definitions() in the top-level CMakeLists.txt, following the same pattern as STORE_USE_ETCD and STORE_USE_REDIS. Also add a message(WARNING) when ENABLE_MULTI_PROTOCOL=ON without USE_CXL=ON since the former requires the latter.

Usage after this change

# CXL memory tier only
cmake -DUSE_CXL=ON ..

# CXL + multi-protocol tiered storage (CXL+TCP or CXL+RDMA)
cmake -DUSE_CXL=ON -DENABLE_MULTI_PROTOCOL=ON ..

# cmake --help-cache will now show both options with descriptions

Impact

• No behavior change when flags are not set (both default to OFF)
• cmake-gui and ccmake will now show these options
• Dependency relationship between the two flags is now explicitly warned about