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/marshal_opts.go

@@ -0,0 +1,25 @@
+package asn1
+
+type marshalOpts struct {
+	slicePreserveTypes bool
+	sliceAllowStrings  bool
+}
+
+// MarshalOpt describes a functional option for marshalling.
+type MarshalOpt func(opts *marshalOpts)
+
+// WithMarshalSlicePreserveTypes preserves the type values from the field parameters when marshaling slices. This is an
+// option since it deviates from stdlib.
+func WithMarshalSlicePreserveTypes(value bool) MarshalOpt {
+	return func(opts *marshalOpts) {
+		opts.slicePreserveTypes = value
+	}
+}
+
+// WithMarshalSliceAllowStrings allows slices of strings when marshaling slices. This is an option since it deviates
+// from stdlib.
+func WithMarshalSliceAllowStrings(value bool) MarshalOpt {
+	return func(opts *marshalOpts) {
+		opts.sliceAllowStrings = value
+	}
+}