CXX-2978 deprecate functor.hpp (#1124)

Warn about future removal

Author: kevinAlbs

CHANGELOG.md

@@ -11,6 +11,10 @@ Changes prior to 3.9.0 are documented as [release notes on GitHub](https://githu
 
 <!-- Will contain entries for the next minor release -->
 
+### Deprecated
+
+- Deprecate use of the `bsoncxx/util/functor.hpp` header.
+
 ## 3.10.2 [Unreleased]
 
 <!-- Will contain entries for the next patch release -->

src/bsoncxx/include/bsoncxx/v_noabi/bsoncxx/util/functor.hpp

@@ -19,6 +19,10 @@
 
 #include <bsoncxx/config/prelude.hpp>
 
+BSONCXX_PRAGMA(
+    message("bsoncxx/util/functor.hpp header is deprecated and will be removed in an upcoming "
+            "major release"))
+
 namespace bsoncxx {
 namespace v_noabi {
 namespace util {