Add a new GET /users/list_instance_emails endpoint that returns all
instance user emails (from the password table). This endpoint is only
available on non-cloud instances.
In the frontend AddUser component, when the instance emails are
available (non-cloud), replace the email text input with a Select
component that allows both selecting from existing instance users and
typing custom emails.
Closes#7902
Co-authored-by: windmill-internal-app[bot] <windmill-internal-app[bot]@users.noreply.github.com>
077eb91c0f