[Issue] extension approval process - post a comment on the PR that tags the azd approvers if an extension needs manual approval

Suggestion from @vhvb1989 - if an extension-registry-update-PR actually requires approval from an azd team member we can post a comment in the PR that tags us so we see it.

Right now our names are printed as part of the CI failure, but that requires the extension developers to have to contact us, whereas this would let us see the notification and know they need help when it happens.

We could deprioritize this, however, because this is a _very_ rare case and might not even happen that often.

Start by locating the extension-registry-update PR workflow and the CI path that currently prints the azd approvers on approval failure. Add a PR comment tagging the relevant approvers when manual approval is required, then verify that ordinary registry updates and already-approved extensions are unaffected.

Written by the indexing model from the issue text.