[v24.2.x] rpk: supports triggering on-demand balancer

[vbotbuildovich]

Backport #22855 to branch v24.2.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 v24.2.x
git checkout -b vbotbuildovich/backport-22855-v24.2.x-24
git cherry-pick -x 62031165e3c6867939e182a4066bf6831088248a

git push origin vbotbuildovich/backport-22855-v24.2.x-24
gh pr create \
  --title "[v24.2.x] rpk: supports triggering on-demand balancer" \
  --base "v24.2.x" \
  --label "kind/backport" \
  --head "vbotbuildovich/backport-22855-v24.2.x-24" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --milestone "v24.2.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/22855 "

JIRA Link: [CORE-7019](https://redpandadata.atlassian.net/browse/CORE-7019)

[daisukebe]

PR is #23216