Add x509proxy library (#5569)

Author: vkuznet

README.md

@@ -281,6 +281,7 @@ _Libraries for implementing authentication schemes._
 - [sessions](https://github.com/adam-hanna/sessions) - Dead simple, highly performant, highly customizable sessions service for go http servers.
 - [sessionup](https://github.com/swithek/sessionup) - Simple, yet effective HTTP session management and identification package.
 - [sjwt](https://github.com/brianvoe/sjwt) - Simple jwt generator and parser.
+- [x509proxy](https://github.com/vkuznet/x509proxy) - Library to handle X509 proxy certificates.
 
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