box: introduce stubs for wal_ext

src/box/CMakeLists.txt

src/box/box.cc

src/box/lua/init.c

src/box/lua/load_cfg.lua

src/box/space.c

src/box/space.h

src/box/space_cache.c

src/box/txn.c

src/box/wal_ext.h

src/trivia/config.h.cmake

test/box-luatest/wal_ext_test.lua