[#3605] Integrate a new fuzzing solution in Kea

The solution is based on clusterfuzzlite, libfuzzer, and oss-fuzz
technologies.

- Add the .clusterfuzzlite directory.
- Add the fuzz CI stage and fuzzing CI jobs.
- Add the fuzzing targets in the fuzz directory.
- Document fuzzing in doxygen.

fuzz/input/fuzz-unix-socket-kea-dhcp4/config-get

@@ -0,0 +1,3 @@
+{
+  "command": "config-get"
+}