feat: add SetType and SetContentType helpers for Token

Adds Token.SetType(typ) and Token.SetContentType(cty) methods as
convenience helpers for setting the 'typ' and 'cty' header parameters
defined in RFC 7519 Sections 5.1 and 5.2.

These methods improve discoverability for registered header parameters
compared to directly mutating Token.Header.

Closes #497

Author: happysnaker

token.go

@@ -100,3 +100,15 @@ func (t *Token) SigningString() (string, error) {
 func (*Token) EncodeSegment(seg []byte) string {
 	return base64.RawURLEncoding.EncodeToString(seg)
 }
+
+// SetType sets the "typ" (Type) header parameter as defined in RFC 7519 Section 5.1.
+// Common values include "JWT", "at+jwt", etc.
+func (t *Token) SetType(typ string) {
+	t.Header["typ"] = typ
+}
+
+// SetContentType sets the "cty" (Content Type) header parameter as defined in RFC 7519 Section 5.2.
+// This is used to declare the media type of the content in nested JWTs.
+func (t *Token) SetContentType(cty string) {
+	t.Header["cty"] = cty
+}