[v22.2.x] pandaproxy: Support SCRAM-SHA-512 for basic_auth

[vbotbuildovich]

Backport #11425 to branch v22.2.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 v22.2.x
git checkout -b vbotbuildovich/backport-11425-v22.2.x-509
git cherry-pick -x 9fc1bcb7b4ec14f43f5f6a85760c6e2d9e1fadf0

git push origin vbotbuildovich/backport-11425-v22.2.x-509
gh pr create \
  --title "[v22.2.x] pandaproxy: Support SCRAM-SHA-512 for basic_auth" \
  --base "v22.2.x" \
  --label "kind/backport" \
  --head "vbotbuildovich/backport-11425-v22.2.x-509" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --reviewer "" \
  --milestone "v22.2.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/11425 "

[BenPope]

basic auth isn't supported in v22.2.x