Allow for disabling all compiler warnings

Unconditionally enabling -Werror is a heavy-handed approach and not
ideal when shipping code to be used by many third parties. For the
sake of minimizing how much I'm changing I'm still ensuring warnings
are on by default but now those building RSL have the option to
disable them entirely and thus also disable -Werror.

CMakeLists.txt

@@ -9,7 +9,8 @@ find_package(rclcpp REQUIRED)
 find_package(tcb_span REQUIRED)
 find_package(tl_expected REQUIRED)
 
-if(CMAKE_CXX_COMPILER_ID MATCHES "(GNU|Clang)")
+option(RSL_ENABLE_WARNINGS "Enable compiler warnings" ON)
+if(RSL_ENABLE_WARNINGS AND CMAKE_CXX_COMPILER_ID MATCHES "(GNU|Clang)")
     add_compile_options(-Werror -Wall -Wextra -Wpedantic -Wshadow -Wconversion -Wsign-conversion -Wold-style-cast)
 endif()