security: Add v2::Validator implementation

Based on the code originally written by Qiuhan Ding

Change-Id: Ib66e24f49d0b6fb2ae21ea1fca7b9ec62ecb753a
Refs: #3289, #1872
diff --git a/docs/specs.rst b/docs/specs.rst
index ca5810b..65803db 100644
--- a/docs/specs.rst
+++ b/docs/specs.rst
@@ -7,3 +7,4 @@
    specs/signed-interest
    specs/certificate-format
    specs/safe-bag
+   specs/validation-error-code