feat(option): add Option type for Varsig construction (initially for varsig < v1)

option.go

@@ -0,0 +1,45 @@
+package varsig
+
+// Options define customization when creating a new Varsig.
+type Options struct {
+	forceVersion0 bool
+	signature     []byte
+}
+
+func newOptions(opts ...Option) *Options {
+	o := &Options{}
+
+	for _, opt := range opts {
+		opt(o)
+	}
+
+	return o
+}
+
+// ForceVersion0 returns a boolean indicating that a Varsig < v1 should
+// be created (which means the encoded Varsig won't have a version field
+// and might contain the signature bytes as the last field.)
+func (o *Options) ForceVersion0() bool {
+	return o.forceVersion0
+}
+
+// Signature returns the optional signature bytes when creating a Varsig
+// < v1.
+func (o *Options) Signature() []byte {
+	return o.signature
+}
+
+// Option is a function that alters the default behavior of constructors
+// that produce implementations of the Varsig type.
+type Option func(*Options)
+
+// WithForceVersion0 indicates that a Varsig < v1 should be produced. If
+// the signature is a) not nil, b) not empty and c) the correct length
+// based on the signing algorithm or signing key, the signature's bytes
+// will be appended to the encoded Varsig.
+func WithForceVersion0(signature []byte) Option {
+	return func(o *Options) {
+		o.forceVersion0 = true
+		o.signature = signature
+	}
+}