Expose repeated extension field as IList

```
To iterate over items of a repeated extensions, one needs to do:

for (int i = 0; i < msg.GetExtensionCount(ExtId); i++)
{
  var ext = msg.GetExtension(ExtId, i);
  // use ext.
}

It would be nice if one could write:

foreach (var ext in msg.GetExtensionList(ExtId))
{
  // use ext.
}

```

Original issue reported on code.google.com by `igorga...@gmail.com` on 7 Mar 2014 at 4:48
