add SetExtraData to CodecParameters to set extradata (#56)

* add SetExtraData

* add simple test case

* review changes

* correct testcase for SetExtraData

codec_parameters_test.go

@@ -95,4 +95,8 @@ func TestCodecParameters(t *testing.T) {
 	require.Equal(t, 4, cp6.SampleRate())
 	cp6.SetWidth(2)
 	require.Equal(t, 2, cp6.Width())
+
+	extraBytes := []byte{0, 0, 0, 1}
+	require.NoError(t, cp6.SetExtraData(extraBytes))
+	require.Equal(t, extraBytes, cp6.ExtraData())
 }