fix: jose.util.base64url.decode returns Buffer (#48096)

According to [their documentation][1] and my testing, the base64url decode
util returns a Buffer, not a string.

https://github.com/cisco/node-jose#uri-safe-base64

types/node-jose/index.d.ts

@@ -353,7 +353,7 @@ export namespace util {
     function randomBytes(len: number): Buffer;
 
     namespace base64url {
-        function decode(base64url: string): string;
+        function decode(base64url: string): Buffer;
 
         function encode(buffer: string | Buffer, encoding?: string): string;
     }