[channelz] Property list: Separate out value conversion from property setting

This will allow open extensibility of the set of types available for property list (we no longer need to add them to the PropertyList type itself), and allow for other shaped types to do the same set of conversions.

PiperOrigin-RevId: 772039595

Author: ctiller

CMakeLists.txt

@@ -9995,6 +9995,7 @@ grpc_cc_library(
 
 grpc_cc_library(
     name = "channelz_property_list",
+    srcs = ["channelz/property_list.cc"],
     hdrs = ["channelz/property_list.h"],
     external_deps = [
         "absl/strings",