[v23.1.x] k/protocol && utils/fragmented_vector improvements

[vbotbuildovich]

Backport #13854 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-13854-v23.1.x-400
git cherry-pick -x 239a9f0979bfb85385eabe869c51e4fd87172326 a2b9966bb9cf471fc4c691dcc9fb0cfa9a554bc6 ceab4007dd8f8bdaf8e6d443869b3808de087be0 3790e24f39b8d9d6c635a2786813a43ef40fd4f7 2c4d1faf116f15a484acf4d2404c901225bae89e ef092e6b959e3faa577f6b5eb48f1c61872bdd73 bc63e9fd0a4905b8c720f3e797406a8399fb0803

git push origin vbotbuildovich/backport-13854-v23.1.x-400
gh pr create \
  --title "[v23.1.x] k/protocol && utils/fragmented_vector improvements" \
  --base "v23.1.x" \
  --label "kind/backport" \
  --head "vbotbuildovich/backport-13854-v23.1.x-400" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --reviewer "travisdowns" \
  --milestone "v23.1.x-next" \
  --body "Backport of PR https:/redpanda-data/redpanda/pull/13854 "

[BenPope]

Fixed by #13897