crypto: add JWT Payload Decoding sample (vlang#23275)

vlib/crypto/README.md

@@ -71,9 +71,11 @@ fn main() {
 	secret := 'your-256-bit-secret'
 	token := make_token(secret)
 	ok := auth_verify(secret, token)
+	pl := decode_payload(token) or { panic(err) }
 	dt := sw.elapsed().microseconds()
 	println('token: ${token}')
 	println('auth_verify(secret, token): ${ok}')
+	println('decode_payload(token): ${pl}')
 	println('Elapsed time: ${dt} uS')
 }
 
@@ -93,4 +95,10 @@ fn auth_verify(secret string, token string) bool {
 	signature_from_token := base64.url_decode(token_split[2])
 	return hmac.equal(signature_from_token, signature_mirror)
 }
-```
+
+fn decode_payload(token string) !JwtPayload {
+	token_split := token.split('.')
+	payload := json.decode(JwtPayload, base64.url_decode_str(token_split[1]))!
+	return payload
+}
+```