Tests/Fuzzing: Add CMake_BUILD_FUZZING option and build infrastructure

Add infrastructure for building fuzz testing targets with libFuzzer or
other fuzzing engines (e.g., OSS-Fuzz's LIB_FUZZING_ENGINE).

Features:
- CMake_BUILD_FUZZING option in root CMakeLists.txt
- Fuzzing/CMakeLists.txt with add_fuzzer() macro
- Support for libFuzzer and external fuzzing engines
- Documentation in Tests/Fuzzing/README.rst

The infrastructure is opt-in and requires a compatible fuzzing engine.
If CMake_BUILD_FUZZING is enabled but no engine is found, configuration
fails with a clear error message.

See Tests/Fuzzing/README.rst for build instructions.