FR: Basic authentication (via ACL)

### What are you trying to do?

Currently we have setup basic authentication inside our application, to show a login screen. However it would be better if the Tailscale agent would be able to do that, because then we can stop untrusted clients before they reach our application.

### How should we solve this?

I am not sure if [this](https://github.com/tailscale/tailscale/pull/1354) pull request can be used somehow to achieve something like that? Perhaps it is totally unrelated.

And also not sure what would be the best way to specify the credentials:
1. Via new agent command line parameters: but then you can only specify a username/password for a single user.
2. Via the ACL somehow. That you can specify multiple username/password combinations. For example for an admin user, and a 'normal' user.

### What is the impact of not solving this?

Now our application is responsible to handle the authentication part. It works but malicious users have reached our application stack, which might contains vulnerabilities. That could be exploited by malicious users to circumvent the application security, and gain access to the application anyway.

### Anything else?

Thanks for taking this into consideration ;-)
Kind regards,
Bart