Add convenience method for getting raw bytes from GetResult

If you want the raw bytes, you can do it today but you need to specify a transcoder that permits getting the content as a byte array, which is verbose. It would be nice if you could just say `contentAsBytes()` like you can with LookupInResult.

Before:

GetResult result = collection.get("foo", GetOptions.getOptions()

        .transcoder(RawJsonTranscoder.INSTANCE));

byte[] contentBytes = result.contentAs(byte[].class);

After:

byte[] contentBytes = collection.get("foo").contentAsBytes();