Bulk and automated invitations from one template or the API

## What

bulk/automated invitations: many single-use links from one template, and create user+invitation via API.

## Want

- Bulk/automated invitations: many single-use links from one template,
- Create user+invitation via API

## Ideas this came from

- [#12742 Allow creation of user and invitations via API](https://github.com/goauthentik/authentik/issues/12742) — Being able to create a user via the API, and generate an invitation would be great to allow for automating the creation of a user, generating an invitation for
- [#20640 Create multiple single-use invite links from the same invitation or invitation template.](https://github.com/goauthentik/authentik/issues/20640) — Is your feature request related to a problem? Every time I create a single-use enrollment invite link, it of course consumes the invite token and deletes the in

The issue names no files, tests, or entry points. Start by reviewing the linked issues #12742 and #20640, then trace the existing invitation and user APIs; done means both bulk single-use links from one template and user-plus-invitation creation are specified and supported.

Written by the indexing model from the issue text.