Add package providers for cares and re2 if not vendored as they won't be a submodule of grpc

Author: raulcd

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -3222,6 +3222,10 @@ function(build_grpc)
     set(_gRPC_CARES_LIBRARIES
         "c-ares::cares"
         CACHE STRING "" FORCE)
+  else()
+    set(gRPC_CARES_PROVIDER
+        "package"
+        CACHE STRING "" FORCE)
   endif()
 
   if(RE2_VENDORED)
@@ -3231,6 +3235,10 @@ function(build_grpc)
     set(_gRPC_RE2_LIBRARIES
         "re2::re2"
         CACHE STRING "" FORCE)
+  else()
+    set(gRPC_RE2_PROVIDER
+        "package"
+        CACHE STRING "" FORCE)
   endif()
   set(gRPC_SSL_PROVIDER
       "package"