FR: auth - control anonymous users using recaptcha (or something else)

### [REQUIRED] Describe your environment

* Operating System version: any
* Browser version: any
* Firebase SDK version: client: 7.6.1 (admin 8.8.0)
* Firebase Product: auth

### Context
The anonymous login is a very important feature for our product and works great :)

However, I could find a recaptcha integration only for phone authentication. (Sorry if I missed something, the docs are extensive).

### Question
Here are some questions:
- We currently could have our DB spammed with unlimited anonymous users, or there is some safeguards in place?
- We wanted a captcha for anonymous users, it is something we can not do at the moment?
- Is there a current alternative to avoid bots / scripts to create unlimited guest users?

### Desired feature
An easy way to control anonymous users registration in Firebase with recaptcha or something else.

Thank you.