KIP-709: batched OffsetFetchRequest (p1)

This is part 1 of KIP-709, which adds the definitions for multiple
groups in OffsetFetchRequest.

This change here fundamentally means we need to change
OffsetFetchRequest to a sharded request internally, which also will
force changes in how sharded group requests are handled / retried.

generate/definitions/09_offset_fetch

@@ -1,15 +1,25 @@
 // OffsetFetchRequest requests the most recent committed offsets for topic
 // partitions in a group.
-OffsetFetchRequest => key 9, max version 7, flexible v6+, group coordinator
+OffsetFetchRequest => key 9, max version 8, flexible v6+, group coordinator
  // Group is the group to fetch offsets for.
- Group: string
+ Group: string // v0-v7
  // Topics contains topics to fetch offets for. Version 2+ allows this to be
  // null to return all topics the client is authorized to describe in the group.
- Topics: nullable-v2+[=>]
+ Topics: nullable-v2+[=>] // v0-v7
  // Topic is a topic to fetch offsets for.
  Topic: string
  // Partitions in a list of partitions in a group to fetch offsets for.
  Partitions: [int32]
+ // Groups, introduced in v8 (Kafka 3.0), allows for fetching offsets for
+ // multiple groups at a time.
+ //
+ // The fields here mirror the old top level fields on the request, thus they
+ // are left undocumented. Refer to the top level documentation if necessary.
+ Groups: [=>] // v8+
+ Group: string
+ Topics: nullable[=>]
+ Topic: string
+ Partitions: [int32]
  // RequireStable signifies whether the broker should wait on returning
  // unstable offsets, instead setting a retriable error on the relevant
  // unstable partitions (UNSTABLE_OFFSET_COMMIT). See KIP-447 for more
@@ -20,7 +30,7 @@ OffsetFetchRequest => key 9, max version 7, flexible v6+, group coordinator
 OffsetFetchResponse =>
  ThrottleMillis(4) // v3+
  // Topics contains responses for each requested topic/partition.
- Topics: [=>]
+ Topics: [=>] // v0-v7
  // Topic is the topic this offset fetch response corresponds to.
  Topic: string
  // Partitions contains responses for each requested partition in
@@ -62,4 +72,18 @@ OffsetFetchResponse =>
  ErrorCode: int16
  // ErrorCode is a top level error code that applies to all topic/partitions.
  // This will be any group error.
- ErrorCode: int16 // v2+
+ ErrorCode: int16 // v2-v7
+ // Groups is the response for all groups. Each field mirrors the fields in the
+ // top level request, thus they are left undocumented. Refer to the top level
+ // documentation if necessary.
+ Groups: [=>] // v8+
+ Group: string
+ Topics: [=>]
+ Topic: string
+ Partitions: [=>]
+ Partition: int32
+ Offset: int64
+ LeaderEpoch: int32(-1)
+ Metadata: nullable-string
+ ErrorCode: int16
+ ErrorCode: int16