rpc: Move signmessage RPC util to new file

Can be reviewed with --color-moved=dimmed-zebra --color-moved-ws=ignore-all-space

ci/test/06_script_b.sh

@@ -38,7 +38,11 @@ if [ "${RUN_TIDY}" = "true" ]; then
   export P_CI_DIR="${BASE_BUILD_DIR}/bitcoin-$HOST/src/"
   CI_EXEC run-clang-tidy "${MAKEJOBS}"
   export P_CI_DIR="${BASE_BUILD_DIR}/bitcoin-$HOST/"
-  CI_EXEC "python3 ${BASE_SCRATCH_DIR}/iwyu/include-what-you-use/iwyu_tool.py src/compat src/init -p . ${MAKEJOBS} -- -Xiwyu --cxx17ns -Xiwyu --mapping_file=${BASE_BUILD_DIR}/bitcoin-$HOST/contrib/devtools/iwyu/bitcoin.core.imp"
+  CI_EXEC "python3 ${BASE_SCRATCH_DIR}/iwyu/include-what-you-use/iwyu_tool.py"\
+          " src/compat"\
+          " src/init"\
+          " src/rpc/signmessage.cpp"\
+          " -p . ${MAKEJOBS} -- -Xiwyu --cxx17ns -Xiwyu --mapping_file=${BASE_BUILD_DIR}/bitcoin-$HOST/contrib/devtools/iwyu/bitcoin.core.imp"
 fi
 
 if [ "$RUN_SECURITY_TESTS" = "true" ]; then