Adds a `Tailscale-Headers-Info` header whenever the `Tailscale-User-`
headers are filled from the HTTP proxy handler.
Planning on hooking this shorturl up to KB docs about the header
values (i.e. what's a login name vs. display name) and security
considerations to keep in mind while using these headers - notibly
that they can also be filled from external requests that do not hit
tailscaled.
Updates https://github.com/tailscale/tailscale/issues/6954
Signed-off-by: Sonia Appasamy <sonia@tailscale.com>
0f5090c526