[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/stringifier.go

@@ -0,0 +1,31 @@
+// Copyright (c) 2019-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package plugin
+
+import (
+	"fmt"
+)
+
+func stringify(objects []interface{}) []string {
+	stringified := make([]string, len(objects), len(objects))
+	for i, object := range objects {
+		stringified[i] = fmt.Sprintf("%+v", object)
+	}
+	return stringified
+}
+
+func toObjects(strings []string) []interface{} {
+	if strings == nil {
+		return nil
+	}
+	objects := make([]interface{}, len(strings))
+	for i, string := range strings {
+		objects[i] = string
+	}
+	return objects
+}
+
+func stringifyToObjects(objects []interface{}) []interface{} {
+	return toObjects(stringify(objects))
+}