[v23.2.x] rpk/bugfix: enable back setting TLS struct values in set commands

[vbotbuildovich]

Backport #13343 to branch v23.2.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 v23.2.x
git checkout -b vbotbuildovich/backport-13343-v23.2.x-322
git cherry-pick -x f8d04d55b482d10bb94bf88a289b7ec639405481

git push origin vbotbuildovich/backport-13343-v23.2.x-322
gh pr create \
  --title "[v23.2.x] rpk/bugfix: enable back setting TLS struct values in set commands" \
  --base "v23.2.x" \
  --label "kind/backport" \
  --head "vbotbuildovich/backport-13343-v23.2.x-322" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --reviewer "twmb" \
  --milestone "v23.2.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/13343 "

[piyushredpanda]

This was done.