[ament_cmake_uncrustify] Add ament_cmake_uncrustify_LANGUAGE variable

This standardizes the interface to manually specify the language for
linters such as cppcheck and uncrustify, such that the necessary CMake
code can look like:

```diff
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f6a8bb3..f71e803 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -19,7 +19,7 @@ if(BUILD_TESTING)

   set(ament_cmake_cppcheck_LANGUAGE c++)
-  set(ament_cmake_uncrustify_ADDITIONAL_ARGS "--language c++")
+  set(ament_cmake_uncrustify_LANGUAGE c++)

 endif()
```

Signed-off-by: Abrar Rahman Protyasha <[email protected]>

ament_cmake_uncrustify/cmake/ament_cmake_uncrustify_lint_hook.cmake

@@ -25,11 +25,20 @@ file(GLOB_RECURSE _source_files FOLLOW_SYMLINKS
 if(_source_files)
   message(STATUS "Added test 'uncrustify' to check C / C++ code style")
 
+  # Forces uncrustify to consider ament_cmake_uncrustify_LANGUAGE as the given language if defined
+  set(_language "")
+  if(DEFINED ament_cmake_uncrustify_LANGUAGE)
+    set(_language LANGUAGE ${ament_cmake_uncrustify_LANGUAGE})
+    message(STATUS "Configured uncrustify language: ${ament_cmake_uncrustify_LANGUAGE}")
+  endif()
+
   set(_args "")
   if(DEFINED ament_cmake_uncrustify_ADDITIONAL_ARGS)
     list(APPEND _args ${ament_cmake_uncrustify_ADDITIONAL_ARGS})
   endif()
 
   message(STATUS "Configured uncrustify additional arguments: ${_args}")
-  ament_uncrustify(${_args})
+  ament_uncrustify(
+    ${_language} ${_args}
+  )
 endif()