LibGemini: Add a response length helper to Gemini::Job

Unlike HTTP responses, Gemini responses do not have a length header or a
delimiter for the end of a response. A response simply continues until
the connection is closed and its length is the number of bytes received.
So, the response length method only returns the result if it has
finished

Userland/Libraries/LibGemini/Job.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <AK/Debug.h>
+#include <AK/Error.h>
 #include <LibCore/Stream.h>
 #include <LibGemini/GeminiResponse.h>
 #include <LibGemini/Job.h>
@@ -223,4 +224,12 @@ void Job::finish_up()
         did_finish(move(response));
     });
 }
+
+ErrorOr<size_t> Job::response_length() const
+{
+    if (m_state != State::Finished)
+        return AK::Error::from_string_literal("Gemini response has not finished");
+
+    return m_received_size;
+}
 }