[v24.3.x] Add record_multiplexer microbenchmarks

[vbotbuildovich]

Backport #24155 to branch v24.3.x. This issue was created as a placeholder because the original PR's commit(s) could not be automatically cherry-picked.
Here are the commands to execute:

git checkout v24.3.x
git checkout -b manual-backport-24155-v24.3.x-937
git cherry-pick -x fe9df92377 2e24c6f203 a0bd72b003 c77ce4bc4a b187703a06 1c29911e06

git push origin manual-backport-24155-v24.3.x-937
gh pr create \
  --title "[v24.3.x] Add `record_multiplexer` microbenchmarks" \
  --base "v24.3.x" \
  --label "kind/backport" \
  --head "manual-backport-24155-v24.3.x-937" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --milestone "v24.3.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/24155 "

JIRA Link: [CORE-8624](https://redpandadata.atlassian.net/browse/CORE-8624)