KAFKA-19160;KAFKA-19164; Improve performance of fetching stable offsets (apache#19497)

When fetching stable offsets in the group coordinator, we iterate over
all requested partitions. For each partition, we iterate over the
group's ongoing transactions to check if there is a pending
transactional offset commit for that partition.

This can get slow when there are a large number of partitions and a
large number of pending transactions. Instead, maintain a list of
pending transactions per partition to speed up lookups.

Reviewers: Shaan, Dongnuo Lyu <dlyu@confluent.io>, Chia-Ping Tsai <chia7712@gmail.com>, David Jaco <djacot@confluent.io>

Author: squah-confluent