[v25.3.x] Add support for PLAIN authn for Shadow Linking

[vbotbuildovich]

Backport #28708 to branch v25.3.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 v25.3.x
git checkout -b manual-backport-28708-v25.3.x-374
git cherry-pick -x 31614c7f68 61430f113e 8acce1791f

git push origin manual-backport-28708-v25.3.x-374
gh pr create \
  --title "[v25.3.x] Add support for PLAIN authn for Shadow Linking" \
  --base "v25.3.x" \
  --label "kind/backport" \
  --head "manual-backport-28708-v25.3.x-374" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --milestone "v25.3.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/28708 "