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/x/encoding/asn1/unmarshal_opts.go

@@ -0,0 +1,25 @@
+package asn1
+
+type unmarshalOpts struct {
+	allowTypeGeneralString bool
+	allowBERIntegers       bool
+}
+
+// UnmarshalOpt describes a functional option for unmarshalling.
+type UnmarshalOpt func(opts *unmarshalOpts)
+
+// WithUnmarshalAllowTypeGeneralString allows the use of ASN.1 DER GeneralString type. This is an option since it
+// deviates from stdlib.
+func WithUnmarshalAllowTypeGeneralString(value bool) UnmarshalOpt {
+	return func(opts *unmarshalOpts) {
+		opts.allowTypeGeneralString = value
+	}
+}
+
+// WithUnmarshalAllowBERIntegers permits the use of ASN.1 BER integer types. This is an option since it deviates from
+// stdlib.
+func WithUnmarshalAllowBERIntegers(value bool) UnmarshalOpt {
+	return func(opts *unmarshalOpts) {
+		opts.allowBERIntegers = value
+	}
+}