Add Nix flake for mciasctl and mciasgrpcctl

Vendor dependencies and expose control program binaries via
nix build. Uses nixpkgs-unstable for Go 1.26 support.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/go-webauthn/webauthn/protocol/extensions.go

@@ -0,0 +1,13 @@
+package protocol
+
+// Extensions are discussed in §9. WebAuthn Extensions (https://www.w3.org/TR/webauthn/#extensions).
+
+// For a list of commonly supported extensions, see §10. Defined Extensions
+// (https://www.w3.org/TR/webauthn/#sctn-defined-extensions).
+
+type AuthenticationExtensionsClientOutputs map[string]any
+
+const (
+	ExtensionAppID        = "appid"
+	ExtensionAppIDExclude = "appidExclude"
+)