How to: generic header auth
other ways to auth by header

if you have a middleware which adds a header with a user identifier, for example tailscale's `Tailscale-User-Login: alice.m@forest.net` then you can automatically auth as `alice` by defining that mapping with `--idp-hm-usr '^Tailscale-User-Login^alice.m@forest.net^alice'` or the following config file:

```yaml
[global]
  idp-hm-usr: ^Tailscale-User-Login^alice.m@forest.net^alice
```

repeat the whole `idp-hm-usr` option to add more mappings