Add convenience method for getting raw JSON bytes from LookupInResult

To retrieve the raw JSON bytes of a lookupIn result item, you can write contentAs(index, byte[].class).

We've seen users struggle to discover this, as in https://forums.couchbase.com/t/return-raw-json-string-from-lookupin/31196/3

Add a `LookupInResult.contentAsBytes(index)` method that returns the raw JSON byte array.

Suggested release note text:

Added a convenience method `LookupInResult.contentAsBytes(int index)` to get the raw JSON bytes of a subdocument.