Secure Tunnel multiplexing support

What's Changed

• X509 in ci by @TwistedTwigleg in #487
• Add the main page to the documentation by @TwistedTwigleg in #535
• GG model update: add client config argument to set async launch mode by @xiazhvera in #525
• Add CMake file for building all the samples at once by @TwistedTwigleg in #537
• Secure Tunnel V2 WebSocket Protocol support by @sbSteveK in #533
• AutoTag PR for v1.21.0 by @TwistedTwigleg in #538

Full Changelog: v1.20.3...v1.21.0