[MM-18898] Stringify plugin.Log* parameters (#12700)

- Add stringutils with method that "stringify" object slices
  - "stringify" means convert each object to its string representation
- Move plugin.Log* implementations to client_rpc, use stringify method before calling server method
- Exclude Log* methods from generated RPC methods
- No signature change for plugin.Log* API
- Add test in plugin_api_test to use plugin.Log* methods with RPC

plugin/interface_generator/main.go

@@ -399,6 +399,10 @@ func removeExcluded(info *PluginInterfaceInfo) *PluginInterfaceInfo {
 			"FileWillBeUploaded",
 			"MessageWillBePosted",
 			"MessageWillBeUpdated",
+			"LogDebug",
+			"LogInfo",
+			"LogWarn",
+			"LogError",
 		}
 		for _, exclusion := range excluded {
 			if exclusion == item {