feat: support transactional consumers in the Reader - #1446

Problem

kafka-go did not implement the client side of Kafka transactions. We found this when building a go service interconnected with NiFi and transactions enabled on the put Kafka processor.

1. Control batches are delivered as messages. Any consumer of a topic written by a
   transactional producer sees empty messages interleaved with real data. The
   protocol requires clients to hide them; kafka-go already parses the Control
   attribute bit into messagesHeader.v2.attributes and never tests it.
2. IsolationLevel: ReadCommitted does nothing (Aborted transactional data included in response when setting read_committed isolation level in reader #1332). The broker bounds
   the fetch at the last stable offset and returns an AbortedTransactions
   list; the client filters. kafka-go parsed that list into a variable it never
   read.

Change

Ports the Java client's CompletedFetch algorithm into messageSetReader, the
single funnel Reader, Batch and Conn all pass through.

Hidden batches consume offsets, so Batch tracks the highest one skipped and
resumes past it.

No exported API is added. dialer.go and consumergroup.go are untouched.
ReadUncommitted stays the default, matching Kafka.

Notes

• Supersedes Skip control message #414, which was closed as "a workaround for a functional gap" with no tests, across 6 files.
• This is the consumer half of Transactions #216's checklist
• closes Aborted transactional data included in response when setting read_committed isolation level in reader #1332
• Complements Sketch of Transaction API #755's producer-side API.
• Overlaps Skip messages from aborted transactions #261, which filtered in the Reader and exported new ControlMessage types; this keeps the filter in the fetch path and adds no API.