KAFKA-20771: Enforce max msg bytes in share group DLQ produce path.

[smjn]

• Add dlqTopicMaxMessageBytes(topic) to
  ShareGroupDLQMetadataCacheHelper,
  honoring a topic-level max.message.bytes override or else falling back
  to the broker's dynamically-reconfigurable message.max.bytes (via
  IntSupplier, not a captured value).
• ShareGroupDLQStateManager now chunks
  an oversized DLQ offset range across multiple produce requests with a
  resumable per-handler integer, and defers (rather than fails) coalesced
  handlers that would push a merged batch over the limit, re-enqueuing
  them for a later round instead of sending oversized requests.
• Also, switches the per-record size check to an incremental calculation
  (DefaultRecord.sizeInBytes) instead of re-serializing the batch-so-far
  on every offset.
• Pins message.timestamp.type=CreateTime on
  auto-created DLQ topics so timestamps aren't silently overwritten by
  the cluster's broker-level default.
• Add new tests and update existing wherever applicable.
• Add integ test to verify the code path.