[v23.1.x] k/p/gen: Use fragmented_vector for fetchable_partition_response

[vbotbuildovich]

Backport #11181 to branch v23.1.x. Note that this issue was created as a placeholder, since the original PR's commit(s) could not be automatically cherry-picked.
Here are the commands to execute:

git checkout v23.1.x
git checkout -b vbotbuildovich/backport-11181-v23.1.x-757
git cherry-pick -x 793d7af15c408ef2ee99cc0c48a95903c3fca19c f33973cd55f9ff1af86dd1e5f484ebfd958ff9b1

git push origin vbotbuildovich/backport-11181-v23.1.x-757
gh pr create \
  --title "[v23.1.x] k/p/gen: Use `fragmented_vector` for  `fetchable_partition_response`" \
  --base "v23.1.x" \
  --label "kind/backport" \
  --head "vbotbuildovich/backport-11181-v23.1.x-757" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --reviewer "travisdowns,StephanDollberg" \
  --milestone "v23.1.x-next" \
  --body "Backport of PR https:/redpanda-data/redpanda/pull/11181 "